GEOLOGY  APPLIED  TO  MINING 


Published   by  the 

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Electrical  World  The  Engineering  and  Mining  Journal 

Engineering  Record  American   Machinist 

Electric  RaiKvay  Journal  Coal  Age1 

Metallurgical  and  Cnemical  Engineering  Power 


GEOLOGY    APPLIED    TO 
MINING 


A   CONCISE   SUMMARY  OF  THE  CHIEF  GEOLOGICAL 
PRINCIPLES,  A  KNOWLEDGE  OF  WHICH  IS  NECES- 
SARY TO  THE  UNDERSTANDING  AND  PROPER 
EXPLOITATION  OF  ORE  DEPOSITS 


FOR  MINING   MEN   AND  STUDENTS 


BY 
JOSIAH  EDWARD  SPURR,  A   M. 

Geologist,  United  States  Geological  Survey ;     Consulting  Geologist  and 

Mining  Engineer  to  the  Sultan  of  Turkey ;  Fellow  of  the  Geological 

Society  of  America;  Member    American    Institute    of    Mining 

Engineers,  Member  Washington  Academy  of  Sciences,  etc. 


FIRST  EDITION  —  SIXTH  IMPRESSION 


McGRAW-HILL  BOOK   COMPANY 
239  WEST  39TH  STREET,   NEW  YORK 

6  BOUVERIE   STREET,  LONDON,  E.G. 


COPYRIGHT,    1904, 
BY 

THE  ENGINEERING  AND  MINING  JOURNAL 
COPYRIGHT,  1907, 

BY    THE 

HILL  PUBLISHING  COMPANY 


PREFACE. 


The  writer  was  led  to  attempt  the  present  volume  througn 
the  perception  of  how  great  a  need  there  was,  among 
mining  men  and  students,  of  some  work  stating  concisely 
those  results  of  the  science  of  geology  which  bear  upon  ore- 
deposits.  No  work  of  this  type  exists,  so  far  as  the  writer 
is  aware,  in  any  language.  The  demand  for  such  informa- 
tion is  great  among  men  of  the  classes  referred  to;  yet  in 
any  of  the  available  works  on  geology  they  find  very  little 
of  that  for  which  they  are  searching,  combined  with  a  great 
deal  of  that  which,  for  the  moment,  is  immaterial. 

In  preparing  this  work  two  points  have  been  kept  in 
mind;  first,  to  make  statements  as  clear  as  possible,  con- 
sidering the  technical  nature  of  the  subject;  and,  second, 
to  present  the  scientific  facts  accurately,  and  as  fully  as 
absolutely  necessary.  Simplicity  of  language  has  been 
constantly  striven  after,  but  it  must  be  remembered  that 
it  is  impossible  to  discuss  any  technical  matter  without 
using  terms  peculiar  to  it. 

This  book,  as  it  goes  forth,  is  far  from  meeting  the 
author's  perfect  approval.  It  is  a  beginning,  and  it  is 

359588 


iv  PREFACE. 

believed  that  the  demand  is  sufficient  to  warrant  its  imme- 
diate publication.  But  it  is  the  writer's  purpose  to  work 
steadily  at  its  improvement  and  elaboration. 

So  sincere  is  his  wish  to  furnish  the  desired  information 
to  the  large  class  for  whom  the  book  is  intended,  that  he 
asks  for  private  communications  from  readers,  stating 
where  they  have  not  found  the  writing  clear  enough,  or 
asking  information  on  questions  not  contained  in  this  book. 
Such  suggestions  will  be  a  valuable  aid  to  future  enlarge- 
ment and  revision. 

Although  the  writer  addresses  men  who  have  had  little  to 
do  with  geology  as  a  science,  or  with  the  theory  of  that  par- 
ticular branch  of  geology  with  which  the  book  deals — 
namely,  the  study  of  ore-deposits — yet,  before  developing 
his  subject,  he  has  deemed  it  necessary  to  anticipate  a  little. 
With  this  purpose  the  first  chapter  has  been  inserted.  For 
a  correct  understanding  of  the  science  of  ore-deposits,  and 
how  the  principles  of  geology  may  be  practically  applied  to 
economic  advantage  in  finding  and  exploiting  ore-bodies,  it 
is  necessary,  first  of  all,  to  have  some  ideas  of  what  ore- 
bodies  are,  and  how  they  have  formed.  The  study  of  the 
processes  of  ore  deposition  has  long  been  in  a  state  of  slow 
growth;  within  the  past  twenty  years,  however,  it  has  been 
more  rapid  and  steady  than  heretofore,  and  the  writer  feels 
justified  in  laying  down  certain  principles. 

Suggestions  and  criticisms  of  the  most  helpful  nature  in 
regard  to  this  work  have  been  made  by  friends,  who  have 
read  portions  of  the  rough  draft  of  the  manuscript.  For 
such  aid  grateful  acknowledgment  is  due  to  Messrs. 


PREFACE.  V 

T.  Wayiand  Vaughan,  A.  H.  Brooks,  and  Waldemar  Lind- 
gren,  of  the  United  States  Geological  Survey.  Especial 
thanks  are  due  to  Mr.  T.  A.  Rickard,  Editor  of  the 
Engineering  and  Mining  Journal,  who  carefully  went  over 
the  manuscript  and  made  such  trenchant  suggestions  as  to 
its  revision  that  the  general  presentation  was  greatly  alter- 
ed and  improved  thereby. 

J.  E.  SPURR. 
Washington,  D.  C.,  Feb.  3,  1904. 


CONTENTS. 


CHAPTER  I. 

THE    PROCESSES    OF   ORE-DEPOSITION. 

Metamorphism,  or  Changes  in  the  Earth's  Crust 1 

The  Origin  of  Metamorphk  Rocks 1 

Transformation  of  Igneous  and  Metamorphic  Rocks  into 

Sedimentary  Rocks 6 

Special  Metamorphic  Processes  Connected  with  Ores     .       7 

Processes  of  Ore-Concentration 9 

Concentration  directly  from  Igneous  Rocks,  while  Molten 

or  Cooling 9 

Theory  of  Direct  Concentration  of  the  Basic  Constit- 
uents, During  a  State  of  Chiefly  Igneous  Fluidity 

of  the  Rock 9 

Theory  of  Concentration  of  the  Silicious  and  other 
Constituents,    in    a    State    of    Aquaeo-Igneous 

Fluidity .      .     13 

Extraction   of  Silicious  and   Other  Constituents   in 
Solution  in  Waters  Expelled  from  Cooling  Rocks, 
and  Deposition  in  Foreign  Rocks     ....     15 
Ore  Deposits  Formed  Chiefly  by  Vapors     ....     17 

The  Origin  of  Certain  Hot  Springs 18 

Concentration  by  Underground  Waters  in  General     .      .     19 

Concentration  by  Surface  Waters     .      .    ' .      .      .      .20 

Relative  Work  of  Underground  and  of  Surface  Waters     21 

The  Mode  of  Ore-Deposition .     22 

CHAPTER  II. 

THE    STUDY    OF    THE    ARRANGEMENT    OF    THE    STRATIFIED    ROCKS    AS 
APPLIED    TO    MINING. 

The  Formation  of  Stratified  Rocks 24 

Formation  of  Sediments  by  Mechanical  Agencies  ...  24 

Formation  of  Sediments  by  Chemical  Agencies  ...  25 

Formation  of  Sediments  by  Organic  Agencies  ....  25 

Transformation  of  Sediments  to  Hard  Rocks  ....  2(5 

The  Physical  Characters  of  Sedimentary  Rocks 26 


VI 11  CONTENTS. 

Page 

The  Chief  Kinds  of  Sedimentary  Rocks,  Their  Origin  and  Char- 
acteristics      28 

The  Distinction  between  Bedding,  Cleavage,  Schistosity,  and 

Gneissic  Structure 32 

Different  Geologic  Periods  during  which  Sedimentary  Rocks 

have  Formed 35 

Characteristics  of  the  Different  Fossils 43 

The  Order  of  Succession  as  Found  in  Actual  Practice     .      .  52 

Relation  of  Physical  Characters  to  Geologic  Age     ...  53 

Comparison  and  Correlation 56 

Mode  of  Determining  the  Relative  Age  of  Different 

Strata         56 

Mode  of  Correlating  Similar  Strata  in  Adjacent  or 

Separated  Regions 57 

The  Association  of  Valuable  Minerals  with  Certain  Strata    .      .  58 

General  Relations  of  Stratified  Ores 58 

Preferential  Association  with  Certain  Geologic  Periods     .  62 
Preferential  Association  with  Certain  Kinds  of  Sediment- 
ary Rocks 68 

Contemporaneous  Deposition  of  Ores  and  Strata     ...  69 
Selection  of  Favorable  Strata  for  the  Subsequent  Deposi- 
tion of  Ores 73 

CHAPTER  III. 

THE   STUDY   OF   IGNEOUS   ROCKS   AS   APPLIED    TO   MINING. 

Physical  Characters  of  Igneous  Rocks     .      .      .      .      .      .      .79 

The  Different  Kinds  of  Igneous  Rocks 82 

Classification  of  Igneous  Rocks  for  Mining  Men 83 

Additional  Definitions 88 

Transitions  between  Different  Kinds  of  Igneous  Rocks     .      .     93 

Forms  of  Igneous  Rocks 95 

General  Relation  between  Igneous  Rocks  and  Ore-Deposits     .     99 
Special  Relation  between  Certain  Igneous  Rocks  and  Ore- 
Deposits      Ill 

Advantages  of  Different  Forms  of  Igneous  Rocks     .      .      .111 

Advantages  of  Different  Kinds  of  Igneous  Rocks     .      .      .112 

Preferences  of  Certain  Igneous  Rocks  for  Certain  Ores, 

.  Displayed  During  the  Cooling  Processes   .      .      .112 
Preferences  of  Certain  Igneous  Rocks  for  Certain  Ores, 
Displayed  by  Selective  Precipitation  of  Metals 
from  Solution 115 


CONTENTS.  IX 

Page 

Ore-bodies  in  the  Role  of  Igneous  Intrusive  Rocks     ....    117 
Igneous  Rocks  Intrusive  Subsequent  to  Ore-Deposition     .      .118 

CHAPTER  IV. 

THE    STUDY    OF    DYNAMIC    AND    STRUCTURAL    GEOLOGY    AS    APPLIED 
TO    MINING. 

Part  I. — General  Conceptions  and  Mapping 120 

Definitions     .  ' 120 

Folds  and  Faults 121 

Effects  of  Erosion  on  Folded  and  Faulted  Rocks       .      .126 

The  Surface  Mantle  of  Debris 130 

The  Systematic  Working  out  of  Geologic  Structure     .      .133 

Strike  and  Dip 133 

Recording  Observations  on  Maps 136 

Migration  of  Outcrops 139 

Construction  of  Geologic  Sections     .      .'•'•-.      .      .      .    142 
Economic  Results  of  Mapping  and  Cross-Sectioning     .    145 
Mapping  and  Sectioning  of  Igneous  Rocks     .      .      .    147 
Part  II. — Rock  Deformation  and  Dislocation,  and  Their  Con- 
nection with  Mineral  Veins 148 

Measurement  of  Folds  and  Faults 148 

Folds  and  Faults  as  Loci  of  Ore-Deposition     .      .      .      .164 

Deposition  of  Ore  in  Folds 164 

Deposition  of  Ore  along  Faults 169 

Joints  in  Rocks 173 

Ore-Deposition  along  Joints 175 

Fractures  and  Fissures 177 

Deposition  of  Ores  along  Fractures  and  Fissures     .      .    184 
Shear-Zones  or  Crushed  Zones,  and  Their  Suitability  for 

Ore-Deposition 193 

General  Relations  Between  Rock  Disturbances  and  Ore- 
Deposits 194 

The    Intersection    of    Circulation    Channels    as    Seats    of 

Mineralization 195 

Rock  Movements  Subsequent  to  Ore-Deposition     .      .      .    197 
Dislocations  Subsequent  to  Ore-Deposition  as  Seats 

for  Later  Mineralization 198 

Ribbon  Structure  199 

Faulted    Faults    and    Their    Relation   to    Ore- 
Deposition     200 

Rock  Movements  along  Earlier-Formed  Dikes       .      .      .   203 


X  CONTENTS. 

Page 

Part  III. — Placers 205 

The  Concentration  of  Gold  in  Placers 205 

Concentration   by   Chemical   Water-Action     .      .      .  206 

Concentration  by  Mechanical  Water-Action     .      .      .  208 

Effects  of  Glacial  Action 211 

Various  Kinds  of  Stream  Gold-Placers 214 

Beach  Placers 218 

Bench  Placers 221 

Old  Placers 222 

Fossil  Placers 226 

Re-concentrated  Placers 227 

Placers  Other  Than  Gold-Placers 229 

Residual  Deposits 233 

CHAPTER  V. 

THE   STUDY   OF    CHEMICAL   GEOLOGY   AS   APPLIED    TO    MINING. 

The  Study  of  Ore-Concentration .      .  235 

The  Shallow  Underground  Waters 237 

The  Work  of  Underground  Waters  in  Dissolving  Rocks     .      .  239 

The  Work  of  Underground  Waters  in  Precipitating  Minerals     .  242 

Manner  of  Deposition  in  the  Deeper  Underground  Regions     .  244 

Special  Chemical  Processes  of  the  Shallow  Underground  Waters  255 

Zone  of  Weathering  or  Oxidation 256 

Precipitation  of  Ores  at  the  Surface 259 

Precipitation  of  Ores  in  the  Shallow  Underground  Zone     .  265 

Concentration  According  to  Relative  Solubilities     .  265 

Secondary  Sulphide  Enrichment •  269 

Features  of  the  Process  of  Reconcentration  of  Pre- 
existing Ores  by  Shallow  Descending  Waters     .  272 
Examples  of  Secondary  Alteration  by  Surface  Waters       .  278 
Manner  in  which  Minerals  are  Precipitated  by  De- 
scending Waters 285 

Characteristics   of   Ore-Deposits   Formed   by   Ascending   and 

by  Descending  Waters 289 

Changes  in  Richness  in  Depth 293 

Association  of  Minerals 299 

Rock  Alterations  as  Guide  to  the  Prospector 301 

CHAPTER  VI. 

THE    RELATION    OF    PHYSIOGRAPHY    TO    MINING. 


ILLUSTRATIONS. 


Fig,  Page 

1.  Map  of  Gap  Nickel  mine,  Lancaster,  Pennsylvania     .      .      .     11 

2.  Ideal  sketch  to  illustrate  unconformities 53 

3.  Cliff  on  Kuskokwim  River,  Alaska,  showing  lateral  tran- 

sition between  •  sandstones  and  shales 64 

4.  Occurrence  of  ore  in  a  definite  stratum,  introduced  subse- 

quent to  the  stratum's  formation.     Rico,  Colorado     .      .     75 

5.  Limestone  beds  of  Derbyshire,  with  intruded  igneous  rock 

traversed  by  veins 76 

6.  Dikes  cutting  granite,  Cape  Ann,  Massachusetts     ...     97 

7.  Primary  pyrrhotite  in  augite 101 

8.  Conditions  in  a  copper  vein  at  Butte,  Montana     ....   116 

9.  Iron-ore  bodies  in  Lola  mine,  Santiago,  Cuba     .      .      .      .118 

10.  Folding  of  limestones  and  shales,  Kuskokwim  River,  Alaska  122 

11.  Close  folding  of  limy  shales,  on  Yukon  River,  Alaska     .      .122 

12.  Overthrown  folds 123 

13.  A  monoclinal  fold 123 

14.  Faults  in  strata  near  Forty  Mile,  Alaska 124 

15.  Reversed  fault  in  Empire  mine,  Grass  Valley,  California     .   124 

16.  Compensating  faults,  Omaha  mine,  Grass  Valley,  California  125 

17.  Eroded  anticlinal  range  of  deformation,  Uinta  Range,  Utah  127 

18.  Simple  fault-scarp  at  the  Palisades,  Yukon  River     .      .      .    129 

19.  Reversed  erosion  fault-scarp  in  the  Lower  Austrian  Alps     .    129 

20.  Bank  of  Glacial  drift,  Gloucester,  Massachusetts     .      .      .132 

21.  Figure  illustrating  strike  and  dip 134 

22.  Symbol  for  recording  strike  and  dip 138 

23.  Diagram  of  a  topographic  base  for  geologic  cross-sections     .    144 

24.  Stereogram  illustrating  the  total  displacement  of  a  fault     .    154 

25.  Stereogram  illustrating  various  functions  of  a  fault   .  .      .   155 

26.  Stereogram  illustrating  the  computation  of  a  fault  move- 

ment, where  part  of  the  data  is  concealed     ....   157 

27.  Stereogram  of  fault,  where  the  lateral  and  perpendicular 

separations  are  zero 158 

28.  Stereogram  illustrating  a  bedding  fault 158 

29.  Ideal  vertical  section  of  faulted  stratified  rocks,  illustrating 

fault  functions      ....  .161 


Xll  ILLUSTRATIONS. 

Fig.  Page 

30.  Diagram  illustrating  the  relations  of  throw  and  vertical 

separation,  in  the  case  of  a  reversed  fault 161 

31.  Diagram  illustrating  the  term  off  set  as  applied  to  a  fault    .    163 

32.  Auriferous  saddle  veins,  Bendigo,  Australia 164 

33.  Diagram  showing  occurrence  of  ore  shoots  in  pitching  arches 

or  folds  of  the  strata,  Elkhorn  mine,  Montana     .      .      .166 

34.  Vein  formation  in  the  fractured  apex  of  an  anticline;  New 

Chum  Railway  mine,  Bendigo,  Australia     ....    167 

35.  Deposition  of  ores  in  anticlinal  folds,  with  barren  synclinals, 

West  Side  Vein,  Tombstone  District,  Arizona     .      .      .168 

36.  Ore-deposition    along    faults,    Bushwhacker-Park    Regent 

mine,  Aspen,  Colorado 170 

37.  Ore-deposition  in  the  fissure  along  a  minor  fault,  Eureka 

vein,  Rico,  Colorado 172 

38.  Columnar  jointing  of  basalt  on  Koyukuk  Mountain,  Yukon 

River,  Alaska 174 

39.  Formation  of  ores  along  joints,  Monte  Cristo,  Washington    »    176 

40.  Sheet  of  glass  cracked  by  torsional  strain 178 

41.  Open  fissure  cutting  and  deflected  by  calcite  vein,  Mercur, 

Utah 180 

42.  Granite  quarry,  showing  increase  of  fractures  and  fissures 

near  the  surface,  Rockport,  Massachusetts 183 

43.  Veins  formed  by  the  successive  selection  of  different  frac- 

tures by  mineralizing  solutions,  Ajax  mine,  Tintic,  Utah  187 

44.  Disappearance  or  deflection  of  veins  on  passing  from  sand- 

stone into  shale,  Bendigo,  Australia 188 

45.  Deflection  of  veins  in    passing    through    slate,    Bendigo, 

Australia 188 

46.  Linked  veins,  Pachuca,  Mexico 192 

47.  Ore  shoot,  Annie  Lee  mine,  Cripple  Creek,  Colorado   .      .      .196 

48.  Ribbon  structure  in  quartz  vein,  Grass  Valley,  California     .   201 

49.  Successive  stages  of  faulting,  Aspen,  Colorado     ....   202 

50.  Vein  following  a  pre-existing  dike,  De  Lamar,  Idaho       .      .   204 

51.  "False  bottom"  of  clay  in  gold  placer  deposit,  Seward  Penin- 

sula, Alaska     210 

52.  Glacier-scooped  basin  containing  auriferous  glacial  gravels, 

Otago  district,  New  Zealand 213 

53.  Irregular  glacier-scooped  depressions,  filled  with  auriferous 

glacial  gravels,  Otago  district,  New  Zealand     ....  213 

54.  Gulch  placer,  Koyukuk  district,  Alaska 215 

55.  Ideal  river,  showing  accumulation  of  auriferous  bars     .      .   217 


ILLUSTRATIONS.  Xlll 

Fig.  Page 

56.  Section  of  beach  placers,  Nome,  Alaska 220 

57.  Bench  and  valley  placers,  Blue  Mountains,  Oregon     .      .   221 

58.  Generalized  section  of  an  old  placer »         223 

59.  Contour  map  of  Neocene  bedrock  surface,  Grass  Valley, 

California         .  225 

60.  Old    auriferous   gravels    (Miocene),   Otago   district,   New 

Zealand 226 

61.  Platinum  placers,  River  Iss,  Ural  Mountains,  Russia     .      .   230 

62.  Section  of  tin  placers,  Siak  district,  Sumatra     ....   231 

63.  Fossil  in  native  silver  as  evidence  of  ore-deposition  by  re- 

placement (of  limestone) 249 

64.  Ore-deposition  by  replacement  of  schist  along  crushed  zone, 

Otago,  New  Zealand 249 

65.  Ore-deposition  at  the  intersection  of  two  circulation  chan- 

nels, Rico,  Colorado          .      .    • 252 

66.  Deposition  of  iron  ore  by  descending  waters  in  joints  and 

pockets  in  limestone,  Pennsylvania  Furnace,  Pennsyl- 
vania     270 

67.  Close  relation  of  galena  zone  to  surface,  evidence  of  depo- 

sition of  descending  waters,  Monte  Cristo, Washington     .   281 

68.  Ore  in  the  roof  formed  by  intersecting  fractures,  as  evidence 

of  deposition  by  ascending  waters,  Bendigo,  Australia     .   291 

69.  Iron-ore  deposit  formed  by  descending  waters,  showing 

constant  relation  to  surface,  Mesabi  range,  Minnesota     .   292 

70.  Gold  in  pyrite  and  quartz.     Thin  section  of  ore  magnified. 

Grass  Valley,  California 300 


CHAPTER  I. 
THE  PROCESSES   OF   ORE   DEPOSITION. 


METAMORPHISM,  OR  CHANGES  IN  THE  EARTH'S 

CRUST. 

THE  ORIGIN  OF  METAMORPHIC  ROCKS. 

7s  the  earth's  surface  stable? 

The  seemingly  stable  crust  of  our  earth  undergoes  slow 
but  stupendous  alterations.  In  the  course  of  our  brief 
lifetime  we  may  not  notice  them;  but,  if  we  do,  we  marvel 
at  them.  Such  things  as  a  river  that  has  shifted  its  course, 
a  harbor  that  becomes  choked  with  sand,  or  a  mud  island 
that  is  washed  away  by  the  waves,  interest  us  strongly. 
Yet  the  researches  of  geology  show  that,  during  the  long 
succession  of  centuries,  rivers  which  run  from  the  uplands 
to  the  sea  may  entirely  remove  mountains  and  spread  them 
out  as  sediments  upon  the  ocean  floor.  In  the  course  of 
time  these  deposits  may  be  lifted  above  the  sea  again  to 
form  new  land;  for  the  crust  of  the  earth  is  not  quiet,  but 
is  forever  heaving  up  and  down,  expanding,  contracting, 
bending  and  breaking,  converting  sea-bottoms  into  dry 
land,  sinking  mountains  into  the  sea,  and  crumpling  plains 
into  mountains.  All  this  goes  on  with  such  undemon- 


2  .          GEOLOGY   APPLIED   TO   MINING. 

strati ve  slowness  that  those  who  live  on  the  earth  are 
hardly  made  aware  of  these  changes  and  are  rarely  dis- 
turbed by  them. 

Example:  Modern  history  records  upward  and  downward 
movements  of  the  land  at  various  points.  It  has  lately 
been  ascertained  that  the  whole  region  of  the  Great  Lakes 
is  undergoing  a  slow  tilting  to  the  south-southwest.  Meas- 
urements, extending  over  a  number  of  years,  of  the  distances 
between  certain  marks  and  the  level  of  the  lakes  render  it 
probable  that  the  region  is  being  lifted  on  one  side  or  de- 
pressed on  the  other,  and  that  the  rate  of  change  is  such  that 
the  two  ends  of  a  line  100  miles  long  and  lying  in  a  south- 
southwest  direction  are  relatively  displaced  four-tenths  of  a 
foot  in  100  years.  The  waters  of  each  lake  are  rising  on  the 
southern  and  western  shores,  or  falling  on  the  northern  and 
eastern  shores,  or  both.  At  Toledo  and  Sandusky,  the 
water  advances  8  or  9  inches  in  depth  in  a  century.  A 
tract  of  land  near  Sandusky  on  which  hay  was  made  in 
1828  is  now  permanently  under  water.  In  3,500  years  the 
Falls  of  Niagara  will  cease  to  flow,  as  a  consequence  of  this 
movement.* 

How  may  sedimentary  rocks  become  metamorphic  ? 

Regions  which  were  once  deeply  buried  may  become  part 
of  the  surface  by  the  removal  of  the  overlying  mass;  and 
study  of  the  rock  thus  revealed  gives  an  idea  of  what  goes 
on  in  the  depths  of  the  earth.  Among  the  lessons  thus 
learned  is  the  following :  When  sediments  have  accumulated 
(as  they  may  in  the  course  of  ages),  to  a  depth  of  several 
miles,  the  lower  layeis  may  be  affected  by  the  weight  of 

*  G.  K.  Gilbert,  18th  Annual  Report  United  States  Geological  Survey: 
Part  II,  pp.  601-645. 


THE  PROCESSES  OF  ORE  DEPOSITION.  3 

those  above,  by  the  internal  heat  of  the  earth  and  other 
causes,  so  that  chemical  changes  take  place.  The  materials 
begin  to  recrystallize,  new  minerals  grow  from  the  debris  of 
those  in  the  sediments;  and  finally  the  rock  becomes  quite 
different  in  appearance. 

Sometimes  we  find  such  a  rock  with  the  marks  of  its 
sedimentary  origin  still  visible.  Other  rocks  may  be  so 
perfectly  recrystallized  that  there  is  no  direct  evidence  in 
their  structure  that  they  ever  were  sediments,  and  we  can 
only  determine  this  point  in  roundabout  ways,  as  by 
tracing  the  much  altered  rock  into  some  less  altered  portion. 
Such  rocks  are  metamorphic;  and  they  are  chiefly  divided 
into  schist  and  gneiss. 


Example:  In  the  northwest  highlands  of  Scotland,  on  Ben 
More  and  on  Sgonnan  More,  movements  in  Cambrian  con- 
glomerates, sandstones  and  shales  have  produced  extraor- 
dinary changes.  The  conglomerate  in  its  unaltered  form  is 
composed  of  rounded  pebbles  in  a  loose,  gritty  matrix. 
Where  subjected  to  movement  the  softer  pebbles  have  been 
crushed,  flattened  and  elongated  in  the  direction  of  move- 
ment. In  some  cases  they  have  been  drawn  out  to  such 
an  extent  as  to  form  thin  lenticular  bands  of  mica  or  horn- 
blende-schist, flowing  around  the  harder  pebbles  of  quartz. 
The  original  gritty  matrix  has  been  converted  into  a  fine 
micaceous  or  chloritic  schist.  Were  it  not  for  the  presence 
of  the  crushed  schistose  pebbles  it  would  probably  be 
impossible  to  tell  that  this  schist  had  a  sedimentary  origin.* 


*  B.  N.  Peach,  J.  Home,  W.  Gunn,  C.  T.  Clough,  L.  Hinxman,  and  H.  M. 
Cadell,  Quarterly  Journal,  Geological  Society,  Vol.  XL1V,  pp.  431-432. 


4  GEOLOGY  APPLIED  TO  MINING. 

How  are  igneous  rocks  formedf 

The  metamorphic  rocks  are  related  to  another  class  of 
crystalline  rocks — the  true  igneous  rocks.  The  igneous  rock 
has  crystallized  from  a  molten  condition.  At  the  surface 
the  formation  of  igneous  rock  is  illustrated  by  lavas,  but  such 
rocks  are  formed  on  a  grander  scale  beneath  the  surface. 
An  igneous  rock  has  generally  a  fairly  constant  texture, 
and  is  composed  throughout  of  the  same  minerals,  which 
are  often  about  the  same  size,  and  lie  in  different  attitudes. 
These  characteristics  arise  from  the  circumstances  that  the 
mass  has  been  fluid  before  cooling,  so  that  all  parts  come  to 
have  about  the  same  composition ;  and  since  all  parts  have 
cooled  under  nearly  the  same  conditions,  the  resulting 
minerals  and  structure  are  the  same. 

Why  are  metamorphic  rocks  often  banded? 

A  true  metamorphic  rock  has  not  been  really  fluid,  in  the 
generally  accepted  sense  of  that  word.  At  the  most,  the 
effect  of  pressure  and  heat  have  made  it  slightly  plastic,  so 
that  it  has  yielded  and  slipped  a  very  little.  Therefore, 
when  it  recrystallized,  the  materials  did  not  move  far  in  the 
rock.  If  there  were  in  the  original  sediments  successive 
vayers  of  different  nature,  (such  as  dark  ferruginous  mud 
beneath  clean  quartz  sand),  the  recrystallized  rock  will 
often  preserve  the  banding;  the  mud  will  appear  as  a  dark 
layer  of  crystalline  ferruginous  minerals  and  the  sand  bed 
will  be  represented  by  crystalline  quartz. 

Banded  structure  in  metamorphic  rocks  may  also  be  pro- 
duced by  more  active  crystallization  along  slipping  planes 
than  in  the  rest  of  the  rock. 


THE  PROCESSES  OF  ORE  DEPOSITION.  5 

May    a   metamorphic    rock    assume    the    characters  of  an 
igneous  rvck? 

The  conditions  which  make  a  mass  plastic  and  those 
which  make  it  fluid  are  not  sharply  separated.  A  rock 
undergoing  metamorphosis  may  become  so  plastic  and  so 
thoroughly  recrystallized  that  the  result  will  be  the  same 
as  if  the  rock  had  slowly  cooled  from  a  molten  state.  Some 
igneous  rocks  are  known  to  have  been  thus  formed,  by  slow 
metamorphism,  from  sediments.  When  we  can  prove  the 
origin  of  such  rocks,  we  often  prefix  the  term  metamorphic 
to  them — thus,  metamorphic  granite — but  often  we  can- 
not tell  whether  a  granite  is  metamorphic  or  igneous,  for 
the  characters  are  alike. 


May  an  igneous  rock  assume  the  characters  of  a  metamorphic 
rock? 

An  igneous  rock  may,  by  becoming  subject  to  conditions 
of  long-continued  slight  plasticity  and  pressure,  acquire  the 
characters  of  a  true  metamorphic  rock.  A  slight  move- 
ment takes  place,  generally  along  close-set  parallel  planes, 
and  here  an  active  recrystallization  and  a  re-arrangement 
of  the  minerals  occur,  resulting  in  a  banded  structure.  The 
rock  may  lose  all  the  traces  of  its  essentially  igneous 
character,  and  become  a  gneiss  or  schist,  indistinguishable 
from  one  that  has  formed  by  the  alteration  of  sediments. 


Example:  The  crystalline  schists   and  gneisses  of  the 
Malvern  Hills,  in  England,  have  been  formed  by  the  meta- 


6  GEOLOGY  APPLIED  TO  MINING. 

morphism  of  igneous  rocks.  Shearing  has  taken  place  in 
bands  of  varying  breadth  situated  at  irregular  intervals. 
The  gneissic  structure  usually  shades  off  on  each  side  of  the 
zone  into  ordinary  igneous  masses  (diorite,  granite,  etc.), 
and  within  the  zone  itself  the  metamorphism  varies  in 
intensity.  Proofs  of  mechanical  forces  resulting  in  shear- 
ing are  numerous.  Hornblende  crystals  are  drawn  out 
into  ribbons,  and  feldspars  are  bent  and  broken.  Fre- 
quently black  mica  is  formed  along  the  shear-planes,  so 
that  the  rock  splits  into  thin  leaves  whose  surfaces  glisten 
with  mica,  while  the  interior  may  be  dioritic.  The  chief 
mineral  changes  are  the  recrystallization  of  feldspar,  and 
the  production  of  biotite,  muscovite,  quartz  and  actinolite.* 


TRANSFORMATION  OF  IGNEOUS  AND  METAMORPHIC  ROCKS 
INTO  SEDIMENTARY  ROCKS. 

Can  igneous  and  metamorphic  rocks  be  changed  back  to  sedi- 
mentary ones? 

The  earth's  surface  consists  in  part  of  igneous  and  meta- 
morphic rocks.  These  rocks  are  attacked  by  the  rain,  the 
sun  and  the  frost;  they  are  broken  up  by  snow,  by  ice,  by 
glaciers,  by  landslides  and  by  the  roots  of  plants;  and  the 
debris  is  carried  down  the  hillsides  into  the  valleys,  and 
along  small  streams  into  large  ones,  till  it  is  emptied  as  sand 
or  mud  into  the  sea,  to  become  slowly  solidified  into  sedi- 
mentary rocks.  The  igneous  or  the  metamorphic  rock 
may  originally  have  been  derived  by  recrystallization  from 

*C.  Galloway,  Quarterly  Journal,  Geological  Society,  Vol.  XLV,  p.  475. 


THE  PROCESSES  OF  ORE  DEPOSITION.  7 

a  sedimentary  one,  so  that  the  materials  have  undergone  a 
complete  circle  or  cycle  of  change. 

Can  we  find  a  beginning  in  the  cycles  of  change? 

So  vast  has  been  the  period  of  time,  during  which  such 
processes  have  been  going  on,  that  there  is  scarcely  any 
rock  of  which  we  can  say  with  certainty  that  it  has  not 
been  derived  from  another,  of  different  nature.  Still,  we 
can  rarely  be  sure  that  an  igneous  rock  has  thus  been 
originated,  and  it  is  probable  that  many  such  rocks  have 
never  been  anything  else,  but  have  crystallized  directly 
from  the  molten  interior.  And  since  we  can  always  trace 
the  sedimentary  and  the  metamorphic  rocks  back  into  the 
igneous  originals — if  we  go  back  far  enough — we  may 
regard  the  igneous  rock  as  the  beginning  of  the  cycle. 

SPECIAL  METAMORPHIC  PROCESSES  CONNECTED  WITH  ORES. 

Has  the  consideration  of  rock-changes  a  direct  bearing  upon 

ore-deposits? 

A  rock  is  an  aggregate  of  minerals;  and  with  the  trans- 
formation of  the  rock  the  minerals  undergo  change.  The 
commonest  rock-forming  minerals  are  quartz,  feldspar, 
mica,  hornblende  and  augite,  these  being  made  up  of  the 
elements  chiefly  represented  in  the  earth's  crust.  The 
rarer  elements  are  also  scattered  through  the  rocks,  and 
occur  in  more  or  less  abundant  minerals.  Certain  of  these 
minerals,  notably  the  heavy  metals,  have  been  put  by  man 
to  use  in  the  arts,  and  it  is  especially  with  these  that  the 
science  of  ore-deposits  is  concerned. 


8  GEOLOGY   APPLIED   TO    MINING. 

What  are  the  limits  of  the  study  of  ore-deposits? 

Accurately  speaking,  the  science  of  economic  geology 
would  embrace  the  study  of  the  distribution  of  all  the 
elements,  for  practically  all  have  some  use;  but  it  is  the 
rarer  ones  that  it  takes  most  intelligence  and  energy  to 
find  and  extract.  For  example,  the  most  common  of  all 
the  elements  (except  oxygen)  is  silicon,  which  in  the  form 
of  sand  is  of  great  economic  value.  But  it  is  so  easy  to  find 
and  dig  sand  that  small  attention  is  paid  to  this  element  in 
the  study  of  Economic  geology.  When  we  come  to  the  next 
commonest  element,  however, — a  metal,  aluminum — we 
begin  to  pay  closer  attention.  Clays,  which  are  impure 
silicates  of  aluminum,  are  sought  after  and  studied  for 
pottery,  porcelain,  brick,  tile,  cement,  etc.;  and  for  the 
manufacture  of  the  pure  metal  and  many  other  purposes  we 
seek  and  investigate  deposits  of  highly  aluminiferous  min- 
erals, such  as  bauxite  and  cryolite,  corundum  and  emery, 
and  natural  alum.  Arriving  at  the  next  commonest  ele- 
ment— iron — we  are  fully  in  the  domain  of  mining;  and  so 
on  down  the  list — calcium,  magnesium,  potassium,  sodium, 
titanium,  carbon,  phosphorous,  manganese,  sulphur, 
barium,  chromium,  nickel,  etc.  The  statement  resolves 
itself  into  this — that  man  finds  artificial  uses  for  all  the 
elements,  and  economic  geology  busies  itself  especially 
with  those  which  are  most  highly  prized,  and  which  are  diffi- 
cult to  find  in  a  sufficient  degree  of  concentration,  or  in  the 
proper  combination  with  other  elements,  forming  minerals 
which  possess  valuable  properties.  Especially  does  it 
require  study  and  effort  to  produce  in  quantities  the  rarer 


THE  PROCESSES  OF  ORE  DEPOSITION.  9 

elements,  notably  the  less  common  of  the  heavy  metals.  On 
that  account  the  science  in  general,  and  this  book  in  par- 
ticular;  will  deal  principally  with  these  metals. 

The  rarer  metals — tin,  lead,  zinc,  silver,  antimony,  gold, 
etc. — occur  in  small  quantities  nearly  everywhere  in  the 
earth's  crust — in  rocks,  in  both  fresh  and  salt  surface 
water,  in  underground  water,  and  even  in  plants  and  in 
animals.  It  requires  rather  exceptional  conditions,  how- 
ever, to  produce  a  mass  containing  such  a  proportion  of 
these  as  to  render  it  profitable  to  make  it  the  basis  of 
mining  operations. 

PROCESSES  OF  ORE-CONCENTRATION. 

In  what  way  does  concentration  of  valuable  elements  take  place? 
We  may  conveniently  divide  the  underground  processes 
of  concentration  into  two  classes — those  which  take  place 
within  igneous  rocks  while  they  are  still  wholly  or  par- 
tially molten  or  during  their  cooling  period,  and  those 
which  are  brought  about  chiefly  through  the  action  of 
percolating  waters  in  solid  rocks. 

CONCENTRATION  DIRECTLY  FROM  IGNEOUS  ROCKS,  WHILE 
MOLTEN  OR  COOLING. 

Theory  of  Direct  Concentration  of  the  Basic  Constituents 
During  a  State  of  Chiefly  Igneous  Fluidity  of  the  Rock. 

How  may  concentration  take  place  in  molten  masses? 

Petrographers  have  advanced  the  theory  that  in  molten 
masses  the  different  elements  tend  to  segregate.  In  this  way 


10  GEOLOGY  APPLIED  TO  MINING. 

it  has  been  supposed  that  different  rocks,  such  as  granite 
and  diabase,  may  separate  out  of  the  same  molten  mass. 
Granite  contains  much  silica,  diabase  much  magnesia  and 
iron. 

Example:  In  west  Cornwall  the  tin  and  copper  veins  are 
associated  with  intrusive  igneous  rocks.  These  are  granites, 
greenstones/  etc.  In  some  cases  it  is  found  that  the  granite 
becomes  less  silicious  toward  the  edges,  a  condition  which 
is  supposed  to  have  been  brought  about  by  segregation 
while  still  molten.  This  granite  is  cut  through  by  more 
silicious  dikes,  which,  however,  are  evidently  closely 
related  to  the  granite.  The  greenstones,  which  are  in 
smaller  quantity,  are  altered  basalts  and  gabbros,  and 
generally  occur  near  the  margins  of  the  granite  intrusions; 
though  not  in  the  granite. 

This  geographical  connection  and  the  order  of  intrusion, 
as  worked  out  for  the  different  rocks,  favor  the  hypothesis 
that  the  silicious  and  the  basic  rocks  have  originated  by  the 
splitting  up  of  an  earlier  molten  mass  of  intermediate  com- 
position.* 

With  the  iron  of  the  basic  rocks  are  generally  small 
amounts  of  some  of  the  less  common  metals,  in  relatively 
greater  quantities  than  in  the  light-colored  silicious  rocks. 
The  metals  are  apt  to  be  more  abundant  in  some  portions  of 
the  dark  heavy  rocks  than  in  others.  Thus  there  may  be 
formed  masses  which  are  chiefly  made  up  of  metallic 
minerals.  By  such  a  process  of  magmatic  segregation 
some  iron  deposits,  some  chromite  (chrome  iron)  deposits, 
some  of  nickel,  etc.,  have  been  supposed  to  be  formed. 

*  J.  B.  Hill,  Transactions  Royal  Society,  Cornwall,  Vol.  XII,  Part  VII,  p.  579. 


THE  PROCESSES  OF  ORE  DEPOSITION. 


11 


Example:  In  Lancaster  county,  Pennsylvania,  the  Gap 
mine  has,  as  chief  metallic  mineral,  magnetic  iron  pyrite 
(pyrrhotite),  which  contains  sufficient  nickel  to  render  it 
valuable  as  an  ore  of  that  metal.  The  ore  occurs  at  the 
contact  of  a  lens-shaped  mass  of  dark  basic  hornblende- 
rock  (amphibolite),  which  has  been  intruded  (thrust  up) 
into  mica-schists.  This  hornblende-rock  is  considerably 
altered  and  when  fresh  had  a  different  mineral  composition, 
being  probably  one  of  the  very  basic*  rocks  gabbro  or  peri- 
dotite.-\  It  is  believed  by  J.  F.  Kempt  that  the  pyrrhotite 
which  occurs  in  the  outer  rim  of  this  basic  intrusion  is  one 
of  the  original  minerals,  crystallized  out  of  the  cooling  rock 
and  segregated  along  the  contact.  (See  Fig.  1). 


Mica  Schist.     Amphibolite.         Granite.  Ore. 

Fig.  1.  Generalized  Map  of  Gap  Mine,  Lancaster,  Pa.,  after  J.  F.  Kemp. 

7s  this  concentration  in  molten  masses  a  very  common  and 

important  process? 

It  is  held  by  many  that  not  infrequently  metals  are  so 
highly  concentrated  in  this  way  as  to  actually  form  ore- 
deposits;  and  that  when  this  is  not  the  case,  the  process  may 

*  The  term   basic   is  applied  to  igneous  rocks  rich  in  dark-colored  heavy 
minerals  containing  iron,  and  poor  or  wanting  in  quartz, 
f  For  definitions  of  these  rocks  see  pp.  86,  90. 
j  "Ore-Deposits  of  the  United  States,"  p.  434. 


12  GEOLOGY  APPLIED  TO  MINING. 

still  be  important,  in  producing  rocks  which  carry  relatively 
large  amounts  of  the  metals,  though  in  a  scattered  condi- 
tion. These  scattered  metals,  through  the  agency  of 
further  concentration,  (by  circulating  waters,  for  example) , 
might  give  rise  to  ore-bodies;  while  the  same  agencies, 
acting  on  rocks,  poor  or  wanting  in  the  metals,  would  not 
contribute  in  that  way. 


Example:  At  Riddle's,  Douglas  county,  Oregon,  there  is  a 
very  basic  rock,  peridotite,  made  up  of  the  minerals  pyr- 
oxene and  olivine.  The  olivine  contains  a  small  percen- 
tage of  nickel,  analysis  having  shown  0.26  per  cent  of  oxide 
of  nickel.  This  rock  has  become  thoroughly  decomposed, 
and  has  altered  to  serpentine.  During  the  alteration  of  the 
olivine,  the  nickel  has  separated  out.  Surface  waters,  per- 
colating through  the  rocks  within  the  zone  of  rock  decay, 
have  taken  the  nickel  into  solut:on,  and  have  precipitated  It 
as  a  coating  on  the  wTalls  of  cracks  and  in  small  veins. 
From  this  method  of  formation  it  has  resulted  that  the 
ores  are  richest  at  the  outcrop,  and  diminish  lower  down, 
till,  on  passing  below  the  zone  of  surface  decay,  they  dis- 
appear.* In  this  case  the  ores  have  been  concentrated  in 
their  present  form  by  surface  waters,  although  in  their 
original  condition  in  the  fresh  rock  they  were  too  sparsely 
scattered  to  be  noticeable;  yet  had  it  not  been  that  there 
was  within  the  molten  rock,  previous  to  cooling,  an  unusual 
proportion  of  nickel,  the  material  would  not  have  been  at 
hand  for  the  waters  to  work  upon,  and  no  ore-deposits 
would  have  been  possible. 


*  Clarke  and  Diller.     American  Journal  of  Science.     Series  iii,  Vol.  XXXV, 
p.  1483. 


THE  PROCESSES  OF  ORE  DEPOSITION.  13 

Theory  of  Concentration  of  the  Silicious  and  Other  Con- 
stituents, in  a  State  of  Aqueo-Igneous  Fluidity. 
It  is  not  so  much  dry  heat  which  renders  molten  igneous 
rocks  fluid,  as  it  is  water  combined  with  heat.  In  the 
different  kinds  of  molten  igneous  rocks,  water  is  present  in 
different  proportions.  In  general,  the  more  silicious  a 
molten  rock  is,  the  more  water  does  it  contain.  The  rela- 
tive order  in  which  the  different  minerals  crystallize  in 
granite,  for  example,  cannot  be  explained  by  dry  heat,  but 
only  by  admitting  that  the  materials  from  which  the 
mineral  formed  were  in  a  state  of  partial  solution  in  water. 
It  is  held  by  some  writers  that  ore-deposits  may  originate; 
under  the  combined  influence  of  water  and  heat,  in  the 
silicious  igneous  rocks. 

How  is  this  process  supposed  to  operate? 

It  has  been  found  in  the  field  that  granites  may  pass 
gradually  into  more  silicious  rocks  composed  of  quartz  and 
feldspar,  and  that  these  may  pass  into  quartz  veins.  It 
has  been  held  by  some  writers  that  such  quartz  veins  have 
a  genetic  connection  with  the  granite.  Their  formation 
has  been  explained  by  applying  the  theory  of  magmatic 
segregation,  as  follows:* 

The  silicious  rocks,  such  as  the  granites,  may  originate 
by  differentiation  from  a  more  basic  magma.  With  the 
further  development  of  this  process,  quartz-feldspar  rocks 
may  be  formed ;  and,  when  the  silica  separates  out  from  the 
magma  in  a  nearly  pure  state,  quartz  veins  may  result. 

*.T  E.  Spurr.  "  Igneous  Rocks  as  Related  to  Occurrence  of  Ores."  Trans- 
actions American  Institute  Mining  Engineers,  Feb.  and  May,  1902,  p.  21. 


14  GEOLOGY  APPLIED  TO  MINING. 

Example:  There  are  numerous  veins  and  large  masses  of 
quartz  throughout  the  district  of  Omeo,  Australia,  in  schists 
or  granular  igneous  rocks.  The  quartz  is  in  places  rnilky 
in  color,  in  others  clear.  In  addition  to  the  quartz  veins 
there  are  others  of  the  same  class  which  contain  tourmaline 
or  feldspar,  or  muscovite  (mica),  or  two  or  all  of  these 
together  in  varying  proportions,  so  that  veins  may  be 
extremely  quartzose  with  but  a  small  proportion  of  other 
minerals,  or  may  be  so  charged  with  them  as  to  become  a 
variety  of  pegmatite.  Study  of  the  veins  composed  of 
quartz  alone,  or  quartz  and  tourmaline,  shows  that  the 
quartz  has  broken  the  tourmaline  crystals,  and  penetrated 
every  crevice.  The  supposition  that  the  quartz  may  have 
been  gradually  deposited  from  solution  around  the  tourma- 
line crystals  till  the  fissure  was  completely  filled  is  negatived 
by  the  observation  that  the  tourmaline  is  not  attached  to 
the  walls  of  the  veins,  but  "floats"  free  in  the  quartz. 
These  facts  are  explained  by  the  author  on  the  hypothesis 
that  the  veins  represent  the  residual  silica  of  the  granitic 
rocks  of  the  region,  after  the  other  minerals  had  crystallized 
out,  and  that  this  residuum  was  squeezed  out  while  in  a 
plastic  state  into  every  adjoining  crevice. 

These  veins  occur  in  a  rich  gold-quartz  region;  never- 
theless, the  author  considers  that  the  auriferous  quartz 
veins  have  had  another  origin.* 

Since  it  is  probable  that  the  amount  of  water  in  the 
igneous  rocks  increases  in  general  with  the  increasing  content 
of  silica,  the  end  product  of  differentiation,  from  which  the 
quartz  veins  are  crystallized,  may  be  little  more  than  highly 
heated  and  compressed  waters,  heavily  charged  with  silica 


*A.  W.   Howitt,    Transactions    Royal    Society    Victoria,  Vol.    XXIII,  pp. 
152-154. 


THE  PROCESSES  OF  ORE  DEPOSITION.  15 

in  solution.  Besides  silica,  other  residual  materials,  left 
over  from  the  magma,  may  be  present,  among  them  gold; 
so  that  the  resulting  quartz  veins  may  be  auriferous.  Such 
veins  might  have  the  same  appearance  as  those  formed  by 
ordinary  underground  waters. 

Example:  In  the  gold-bearing  district  of  Silver  Peak, 
Nevada,  are  found  quartz  veins  which  pass  gradually  into 
silicious  granitic  dikes,  and  seem  to  represent  the  silicious 
extreme  of  segregation  or  differentiation  of  the  granite. 
Such  veins  usually  contain  a  little  feldspar  and  white  mica, 
while  others  to  which  a  similar  origin  may  be  assigned,  con- 
tain none.  Assays  for  gold  and  silver  were  made  from  two 
of  these  veins.  One  contained  0.03  oz.  gold  and  0.13  oz. 
silver,  the  other  none.* 

Extraction  of  Silicious  and  Other  Constituents,  in  Solution, 

in  Waters  Expelled  from  Cooling  Rocks,  and 

Deposition   in   Fore'gn   Rocks. 

Are  waters  and  vapors  active  when  an  igneous  rock  is  in 

process  of  cooling? 

When  an  igneous  rock  begins  to  cool  and  harden,  much 
water,  which  has  been  a  part  of  the  molten  material  but 
cannot  form  part  of  the  rock,  is  pressed  out.  If  the  igneous 
rock  is  at  the  surface,  like  a  lava,  this  water,  which  is  highly 
heated,  passes  off  in  copious  and  long-lasting  clouds  of 
steam.  In  the  case  of  necks  of  molten  rock,  which  feed 
volcanoes,  and  of  other  bodies  which  have  forced  their 


*  H.  W.  Turner.     Report  for  United  States  Geological  Survey.     (Unpub- 
lished MSS.) 


16  GEOLOGY  APPLIED  TO  MINING. 

way  up  from  the  depths,  through  other  rocks,  but  have 
not  succeeded  in  getting  to  the  surface,  the  water  is 
forced  into  the  adjoining  rocks  and,  being  under  pressure, 
may  be  either  in  liquid  or  vaporous  form. 

These  waters  are  highly  charged  with  various  strong 
vapors,  and  both  water  and  vapors  carry  in  solution  much 
mineral  matter,  among  which  may  be  the  metals.  It  is 
believed  by  many  geologists  that  this  mineral  matter  is 
deposited  while  the  solutions  are  in  process  of  circulation 
through  the  rocks,  and,  further,  that  the  metals  may  be 
concentrated  so  as  to  form  ore-deposits.  Certain  ore- 
bodies  found  at  the  contact  of  an  igneous  rock  with  another 
rock  have  been  described  as  having  this  origin.  Such 
occurrences  are  termed  contact-metamorphic  ore-deposits. 

The  commonest  kind  of  contact  metamorphic  ore-de- 
posit is  usually  held  to  occur  at  the  very  contact  of  the 
igneous  rock.  But  contact  metamorphism  in  general  may 
extend  much  further,  forming  an  altered  zone  a  mile  or 
more  broad.  Anywhere  within  this  zone  deposits  of 
metallic  minerals,  with  the  characteristics  of  contact- 
metamorphic  ore-deposits,  may  be  found. 

Example:  The  Dolcoath  mine,  in  the  Elkhorn  mining  dis- 
trict, Montana,*  lies  in  limestone,  at  a  distance  of  over  half 
a  mile  from  the  granite,  which  has  chiefly  occasioned  the 
metamorphism  of  the  district.  Through  this  metamor- 
phism the  limestones  have  been  recrystallized  to  marble, 
the  sandstones  have  become  quartzites,  the  sandy  and  limy 
shales  are  largely  recrystallized  to  new  minerals  such  as 

*W.  H.  Weed,  22d  Annual  Report  United  States  Geological  Survey,  Part  II, 
p.  506. 


THE  PROCESSES  OP  ORE  DEPOSITION.  17 

pyroxene,  garnet,  epidote,  etc.  The  ore-bearing  stratum 
of  the  mine  was  originally  a  bed  of  impure  limestone,  which 
has  been  metamorphosed  to  garnet  and  pyroxene,  with 
spots  of  calcite.  Associated  with  these  gangue-minerals  are 
sulphide  and  telluride  of  bismuth,  containing  gold. 


Ore-Deposits  formed  Chiefly  by  Vapors. 

May   some    ore-concentrations   be   accomplished   chiefly   by 

vapors? 

Tin-veins  are  held  by  many  writers  to  be  usually  formed 
in  this  way.  They  are  ordinarily  confined  to  granite.  The 
explanation  usually  offered  is  that  when  the  granite  cools, 
it  shrinks,  and  crevices  begin  to  open.  Water  escaping 
from  the  hardening  rock  rises  along  these  rents  in  the 
form  of  vapor,  and  is  accompanied  by  other  especially 
powerful  vapors,  such  as  chlorine  and  fluorine.  The 
vapors  may  carry  tin  and  other  mineral  matters,  which 
they  may  deposit  in  the  rents  or  in  the  porous  walls,  and 
thus  concentrate  them  sufficiently  to  make  an  ore-deposit. 
Are  tin-veins  alone  due  to  he  action  of  vapors  f 

Others  motals  are  known  to  be  deposited  by  escaping 
vapors.  They  have  been  found  encrusting  the  mouths  of 
steam- jets  (fumaroles)  in  lavas.  Cinnabar,  the  ore  of  mer- 
cury, and  realgar,  an  ore  of  arsenic,  as  well  as  hematite,  an 
ore  of  iron,  with  copper  and  lead  chlorides,  are  among  the 
metallic  minerals  which  have  been  thus  deposited  at 
Vesuvius.  It  is  likely  that  some  workable  cinnabar  de- 
posits and  even  some  of  the  other  metals  may  have 
been  formed  underground  by  vapors  alone. 


18  GEOLOGY  APPLIED  TO  MINING. 

The  Origin  of  Certain  Hot  Springs. 

What  becomes  of  the  water  expelled  from  molten  rock  in  cooling, 
besides  that  which  passes  off  in  vapors  at  the  surface? 
We  have  seen  that  when  intensely  heated  rock  cools  at  the 
surface,  great  quantities  of  the  expelled  waters  pass  off  as 
clouds  of  steam.  As  the  crust  slowly  hardens,  and  the  con- 
gealing molten  rock  becomes  further  away  from  the  surface, 
the  escaping  waters  will  become  cooler  in  their  passage 
upward.  A  stage  will  finally  be  reached  when  they  will  not 
entirely  flash  into  steam  on  emerging,  but  will  remain 
liquid,  though  boiling  and  sending  off  a  great  deal  of  steam. 
They  will,  in  fact,  emerge  as  hot  springs,  and  it  is  probable 
that  the  change  from  steam-jets  (fumaroles)  to  hot 
springs  is  the  normal  process  of  cooling  volcanoes.  As  the 
cooling  progresses,  these  springs  will  lose  in  temperature, 
volume  and  pressure,  until  finally  they  will  in  many  cases 
become  extinct. 

The  water  which  is  given  off  at  the  contact  of  an  intrusive 
mass  of  igneous  rock,  and  which  is  frequently  so  active  in 
producing  contact-metamorphism,  must  also  exist  after  it 
has  accomplished  these  changes.  We  may  suppose  that  if 
there  are  any  channels,  such  as  are  afforded  by  fissures  or 
faults,  this  water  may  find  its  way  upward,  and  perhaps 
even  reach  the  surface.* 

May  such  waters  produce  ore-deposits? 

We  have  seen  in  considering  contact-metamorphic  ore- 

*  Springs  having  this  origin  may  be  called  (following  Professor  Suess,  of 
Vienna,)  juvenile  springs,  the  term  referring  to  the  recent  birth  of  the  water 
from  the  molten  rock. 


THE  PROCESSES  OF  ORE  DEPOSITION.  19 

deposits  that  compressed  vapors  and  waters  expelled  from 
solidifying  igneous  bodies  are  supposed  to  produce  ores  in 
the  adjacent  intruded  rocks;  and  that  the  vapors  that 
escape  from  volcanoes  often  deposit  metallic  minerals. 
Therefore  it  may  well  be,  also,  that  the  hot  waters  which 
succeed  the  vapors  in  the  cooling  of  volcanic  rock  are 
efficacious  in  concentrating  ores.  Ascending  hot  waters 
are  generally  conceded  to  be  the  most  powerful  agents  of 
mineralization,  and  those  hot  waters  which  have  the  origin 
above  described  should  be  especially  active,  for  in  addition 
to  their  dissolving  power,  exerted  on  rocks  which  they 
traverse,  they  may  contain  metals  expelled  in  solution  in 
them  from  the  crystallizing  rocks  from  which  they  have 
emanated. 

CONCENTRATION  BY  UNDERGROUND  WATERS  IN  GENERAL. 

Does  concentration  cease  when  the  rock  is  cold? 

The  work  of  concentration  does  not  cease  with  the  com- 
plete cooling  and  hardening  of  the  igneous  rock.  Rain- 
water, falling  upon  the  surface,  is,  in  part,  carried  off  in 
rivulets  and  streams  to  the  ocean ;  but  probably  the  greater 
part  sinks  below  the  surface.  The  underground  water  cir- 
culates chiefly  through  natural  channels,  such  as  are  offered 
by  any  fissure  or  porous  zone;  but  it  also  possesses  the 
power  of  working  itself  very  slowly  through  most  solid 
rocks.  From  the  moment  these  waters  touch  the  surface 
they  dissolve  substances  from  the  rocks  and  precipitate 
them  again  at  other  points.  This  work  they  do  continu- 
ously, and  thus  as  far  down  as  they  penetrate  there  is  a 


20  GEOLOGY  APPLIED  TO  MINING. 

constant  shifting  of  material.  From  the  affinity  of  like 
minerals  for  each  other,  this  shifting  results  in  concentra- 
tion; and  where  metallic  minerals  are  concentrated,  ore- 
deposits  are  formed. 

These  waters,  after  sinking  deeply,  or  nearing  some  body 
of  hot  igneous  rock,  may  be  supposed  to  become  heated, 
and  would  then  be  still  more  powerful  than  before.  They 
may  take  up  the  unfinished  work  of  concentration  left  by 
the  cooling  processes  of  igneous  rocks,  and  carry  it  to  a 
successful  finish  in  the  form  of  a  workable  body  of  ore;  or 
they  may  concentrate  the  metals  sparsely  scattered  through 
igneous,  sedimentary  or  metamorphic  rocks. 

7s  there  any  universal  final  stage  of  concentration? 

These  processes  of  concentration  are  never  at  an  end. 
With  changing  currents  of  water  the  ores  are  redissolved 
and  reprecipitated,  changing  their  position  and  proportion. 
An  ore-body  formed  by  deep  underground  waters  may,  in 
consequence  of  the  slow  wearing  away  of  the  surface,  finally 
come  to  be  exposed,  or  " outcrop."  Then  the  shallow  under- 
ground waters  may  either  make  the  ore  poorer,  or  make  it 
richer,  by  dissolving  and  reprecipitating. 

Even  after  a  mine  is  opened,  the  work  goes  on,  and 
metals  are  often  deposited  on  the  walls  of  drifts,  or  encrust 
tools  which  may  be  left  in  old  workings. 

CONCENTRATION  BY  SURFACE  WATERS. 

Surface  waters  have  a  twofold  effect — chemical  and 
mechanical. 


THE  PROCESSES  OF  ORE  DEPOSITION.  21 

How  do  surface  waters  act  mechanically  so  as  to  concentrate 

ores? 

In  large  bodies  of  surface  waters,  as  in  streams  or  on  the 
shores  of  the  ocean,  the  sediments,  or  finely  ground  mate- 
rials worn  from  the  rocks,  become  arranged  according  to 
the  relative  weight  and  size,  by  the  operation  of  the  same 
laws  as  those  by  which  ores  are  concentrated  in  mills.  In  a 
current  of  water  the  heaviest  minerals  sink  first,  and  so  are 
separated  from  the  lighter  material,  which  is  carried  on. 
When  the  sediment,  which  is  thus  transported  by  water, 
has  been  taken  from  a  decomposing  rock  containing  valua- 
ble minerals,  such  as  gold,  platinum  and  tin,  these  heavy 
minerals  become  concentrated  at  certain  points  where  the 
current  is  too  weak  to  carry  them  further,  but  is  too  strong 
to  allow  most  of  the  other  materials  to  drop. 

How  do  surface  waters  act  chemically  in  concentrating  ores? 

There  is  no  stream,  however  clear,  which  does  not  contain 
dissolved  mineral  matter.  This  material  may,  on  occasion, 
be  precipitated  in  large  quantities,  making  sometimes  a 
deposit  of  economic  value. 

RELATIVE  WORK  OF  UNDERGROUND  AND  OF  SURFACE 
WATERS. 

How  do  the  mechanical  and  chemical  activities  of  underground 

waters  compare  with  those  of  surface  waters? 

Underground  waters  move  slowly  through  the  rocks, 

often  occupying  every   available  space,   no  matter  how 

minute.     Ordinarily,    however,    they    cannot    unite    into 

bodies  of  large  volume  like  rivers  and  lakes,  for  they  cannot 


22  GEOLOGY  APPLIED  TO  MINING. 

find  underground  spaces  large  enough.  This  difference 
makes  their  mechanical  power  practically  nothing — for 
they  cannot  carry  mineral  particles  with  them  by  the  force 
of  their  motion.  But  their  chemical  work  is  vastly  more 
important,  for  two  chief  reasons.  The  first  is  the  greater 
field  of  the  underground  waters,  which  work  up  and  down, 
and  through  and  through  a  thick  belt  of  rocks  containing 
small  quantities  of  metals,  while  the  surface  waters  only 
skim  the  top  of  this  belt.  The  second  reason  is  that,  by 
virtue  of  the  pressure  and  heat  which  the  underground 
waters  frequently  attain,  their  power  of  solution,  and  hence 
of  concentration,  is  correspondingly  increased. 

THE  MODE  OF  ORE  DEPOSITION. 
Are  ore-bodies  formed  by  upward,  downward,  or  laterally 

moving  waters? 

Underground  waters  move  sometimes  upward,  some- 
times downward,  sometimes  sidewise;  and,  whatever  their 
direction,  they  have  the  power  to  dissolve  and  reprecipitate 
mineral  matter  and  hence  to  bring  about  concentrations  of 
ore.  The  ore-deposit  formed  by  descending  waters  may 
often  be  with  difficulty  distinguished  from  one  formed  by 
ascending  waters.  Yet  from  the  fact  that  heated  waters 
naturally  rise,  and  that  they  are  more  capable  of  solution 
than  cold .  ones,  it  is  probable  that  the  most  important 
single  class  of  ore-deposits  has  been  formed  by  them. 

How  are  ores  deposited  by  waters  in  rocks f 

Following  the  question  as  to  what  kind  of  underground 
water  has  accomplished  ore-deposition,  the  next  inquiry 


THE  PROCESSES   OF  ORE  DEPOSITION.  23 

• 

concerns  the  manner  in  which  the  ores  in  solution  are 
deposited  in  the  rocks.  The  theories  advanced  by  learned 
men  have  perhaps  exhausted  all  the  possibilities  of  the 
imagination  as  well  as  of  reason.  Three  chief  theories,  now 
each  of  them  proved  facts,  have  been  most  successful  in 
standing  the  test  of  time.  These  are  the  theories  of  substi- 
tution or  replacement,  of  cavity- filling,  and  of  impregnation  or 
the  filling  of  interstices  (interstitial  filling). 

Study  shows  that  not  one  process  is  represented  in  the 
average  ore-body,  but  many.  In  most  of  them,  one  may 
find  excellent  examples  of  the  work  of  each  of  the  three 
processes  above  mentioned,  and,  even  in  a  single  hand- 
specimen  of  ore,  the  same  multiplicity  of  origin  may  be 
displayed;  although  in  general  one  process  is  chiefly  active 
in  forming  a  certain  ore  deposit,  and  another  process  in  a 
second.  Thus  we  have  many  typical  replacement  deposits 
(among  them  many  lead-silver  ore-bodies  in  limestones) 
and  many  typical  fissure  veins  (where  an  open  rift  or  fissure 
has  been  filled  by  ore) ;  yet,  in  the  replacement  deposit,  one 
may  often  find  instances  of  fissure-filling;  and,  in  the  fissure 
vein,  examples  of  replacement. 


CHAPTER  II. 

THE  STUDY  OF  THE  ARRANGEMENT  OF  STRATI- 
FIED ROCKS  AS  APPLIED  TO  MINING. 


THE  FORMATION  OF  STRATIFIED  ROCKS. 

FORMATION  OF  SEDIMENTS  BY  MECHANICAL  AGENCIES. 
How  are  sediments  formed  by  mechanical  agencies? 

Rivers  come  down  from  their  sources  laden  with  mud  and 
dragging  along  pebbles  on  their  bottoms;  on  reaching 
the  sea  the  coarse  gravel  is  usually  deposited  near  the 
mouths  of  the  rivers,  while  the  finer  material  is  carried 
further  on.  Along  the  shore  the  waves  attack  the  cliffs, 
undermine  them  and  finally  cause  them  to  break  off,  and 
in  this  way  new  supplies  of  rock  are  produced,  to  be  ground 
into  sand  and  mud  by  the  churning  of  the  surf.  The 
tides  and  currents  sweep  the  material  far  out  to  sea  or  along 
the  coast. 

In  lakes,  the  material  brought  down  by  the  streams  like- 
wise settles  on  the  bottom. 

Rivers  work  slowly  to  either  side  in  their  valleys;  they 
nearly  always  have  a  winding  course,  and  at  every  curve 
the  current  may  be  seen  cutting  under  and  removing  the 
bank  on  the  concave  side,  and  depositing  sediment  on  the 
opposite  or  convex  margin,  so  as  to  produce  a  spit  or  bar. 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  25 

The  result  of  many  centuries  of  this  cutting  and  rebuilding 
is  that  the  stream  widens  the  valley,  and  that  the  valley 
becomes  covered  with  gravel  and  sand,  which  has  been 
washed,  worked  over,  and  abandoned  by  the  stream. 

FORMATION  OF  SEDIMENTS  BY  CHEMICAL  AGENCIES. 
Are  all  sediments  formed  by  broken  and  ground-up  rock? 

Besides  the  material  which  surface  waters  carry  in  sus- 
pension, as  mud,  sand,  or  gravel,  they  contain  substances 
in  solution.  In  limestone  districts,  for  example,  the  waters 
contain  lime,  and  cooking  utensils  and  boilers  in  which 
such  water  is  used  become  coated  with  this  material,  depos- 
ited from  the  evaporating  liquid.  In  nature  this  lime  is 
deposited  similarly  and  on  a  large  scale.  In  shallow  lakes 
and  land-locked  seas  the  water  brought  down  by  streams 
from  limestone  regions  may,  after  standing  and  evaporating, 
precipitate  lime  on  the  bottom  of  the  lake.  This  deposit  is 
generally  lime  carbonate  (limestone) ;  sometimes  it  is  lime 
sulphate  (gypsum). 

Is  lime  the  only  material  chemically  precipitated  as  sediment 
in  such  cases? 

Besides  lime,  other  m'nerals  are  chemically  precipitated 
in  ocean  and  lake  waters,  especially  silica,  but  all  in  a  far 
less  degree. 

FORMATION  OF  SEDIMENTS  BY  ORGANIC  AGENCIES. 
In  lakes,  seas  and  in  the  ocean,  there  live  myriads  of 
animals  that  form  their  hard  parts  by  extracting  it  from  the 
sea  water.    They  absorb  the  mineral  that  is  in  solution  and 


26  GEOLOGY  APPLIED  TO  MINING. 

build  it  into  their  shells.  It  is  generally  lime  that  they 
absorb  and  their  shells  are  of  lime  Carbonate.  Such  are  all 
of  our  ordinary  shell-fish,  as  well  as  corals  and  a  myriad  of 
others,  familiar  and  unfamiliar.  It  is  a  familiar  story  how 
corals  live  and  die,  leaving  their  limy  shells  behind;  how 
new  animals  build  upon  the  skeletons  of  their  ancestors, 
and  so  on,  till  great  masses  are  produced.  In  the  same 
way  other  shell-bearing  marine  animals  may  furnish 
material  which,  little  by  little,  accumulates  to  great 
thickness.  There  are  thick  strata  which  consist  almost 
entirely  of  oyster  shells,  and  so  on. 

TRANSFORMATION  OF  SEDIMENTS  TO  HARD  ROCKS. 
How  do  these  sediments  become  rocks  and  dry  land? 

By  the  warping  and  folding  of  the  earth's  crust,  brought 
about  slowly  during  centuries  of  centuries,  sediments  are 
brought  out  of  the  water  and  become  part  of  the  land. 
They  ordinarily  harden  with  time.  They  may  come  to 
occupy  any  position;  it  is  as  common  to  find  sedimentary 
rocks  on  the  top  of  mountains  as  in  the  low  plains. 

THE  PHYSICAL  CHARACTERS  OF  SEDIMENTARY 
ROCKS. 

How  can  one  distinguish  sedimentary  rocks  from  metamorphic 

or  igneous  rocks? 

Sedimentary  rocks  are  distinguished  from  metamorphic 
or  igneous  rocks  by  their  physical  characters.  They  are 
often  plainly  fragmental — that  is,  they  are  made  up  of 
broken,  often  waterworn  fragments,  large  or  small;  or, 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  27 

like  limestones,  they  are  of  a  composition  unknown  in  any 
other  than  sediments.  Fossils  are  almost  infallible  evi- 
dences of  the  sedimentary  origin  of  the  rock  which  contains 
them. 

What  is  stratification  or  bedding? 

This  is  another  feature  of  sedimentary  rocks.  When  we 
cut  a  pit  in  the  sand  on  the  sea-shore,  we  see  that  the 
material  is  deposited  in  layers,  one  layer,  for  instance, 
being  sand  and  another  pebbles;  or,  if  all  the  layers  are  of 
sand,  there  is  some  slight  difference,  as  of  color. 

This  arrangement  of  successive  layers  is  called  bedding 
or  stratification.  It  arises  from  the  fact  that  the  material 
laid  down  in  water  will  ordinarily  vary  during  successive 
periods.  All  sedimentary  rocks  show  this  characteristic 
more  or  less  plainly.  Each  separate  layer  is  called  a  bed 
or  stratum.  Some  rocks  showr  distinct  beds  only  a  few 
inches  or  even  a  fraction  of  an  inch  thick ;  these  rocks  have 
been  deposited  under  changing  conditions — near  the  shore 
of  an  ocean,  for  example,  where  the  varying  currents 
brought  about  supplies  of  different  kinds  of  detritus.  In 
other  rocks  the  beds  are  thicker,  and  in  some  they  may 
extend  through  a  thickness  of  many  feet  with  a  scarcely 
perceptible  stratification. 

What  is  meant  by  the  word  '  formation '  as  used  in  reference 

to  rocks? 

The  term  formation  is  generally  used  by  miners  as  a 
name  for  any  particular  body  of  rock — thus  a  limestone 
formation,  etc.  This  use  of  the  word  is,  in  the  writer's 


28  GEOLOGY   APPLIED  TO    MINING. 

opinion,  proper.  Geologists  use  the  word  with  a  different 
technical  meaning,  limiting  its  use  in  various  ways. 
Where  rocks  of  different  kinds  and  of  different  ages 
lie  one  over  the  other,  each  belt,  marked  by  certain  con- 
stant characteristics,  is  called  a  formation.  In  this  sense 
a  formation  may  be  only  a  few  feet  or  thousands  of  feet 
thick.  Geologists  often  give  a  distinctive  name  to  each 
formation,  for  the  sake  of  identifying  it  in  description  or 
in  mapping. 

THE   CHIEF    KINDS    OF   SEDIMENTARY   ROCKS, 
THEIR  ORIGIN  AND  CHARACTERISTICS. 

What  are  the  ordinary  sedimentary  or  stratified  rocks? 

The  ordinary  sedimentary  rocks  are  conglomerate,  grit, 
sandstone,  quartzite,  shale,  slate,  limestone,  marble  and 
dolomite. 

What  are  conglomerates,  and  what  is  their  origin? 

Beds  of  pebbles,  when  cemented  together,  form  con- 
glomerate. They  can  be  distinguished  by  the  stratification 
and  the  rounded  pebbles. 

What  is  a  grit? 

A  coarse  and  impure  sand,  when  hardened,  is  called  a 
grit. 

What  is  a  sandstone,  and  how  does  it  originate? 

When  sand  hardens  so  that  the  grains  stick  firmly 
together  it  becomes  a  sandstone.  Sandstone  is  of  all 
colors,  white  or  red  being  the  most  frequent.  In  pure 


ARRANGEMENT    OP    STRATIFIED    ROCKS.  29 

sandstone  the  component  grains  are  entirely  of  quartz; 
these  rounded  grains  can  usually  be  seen  with  the  naked 
eye.  Sandstones  are  porous,  for  between  the  grains  there 
are  tiny  spaces  or  interstices. 

How  does  quartzite  originate? 

The  underground  waters  which  usually  permeate  sand- 
stones frequently  bring  silica,  which  they  deposit  in  the 
pores  between  the  sand  grains.  After  a  period  there  may 
result  a  solid  quartz  rock,  or  quartzite. 

How  can  one  tell  quartzite  from  vein  quartz? 

Sometimes  quartzite  is  pure  white,  and  very  difficult  to 
distinguish  from  vein  quartz,  which  has  been  deposited 
entirely  from  underground  waters.  A  close  scrutiny, 
especially  with  a  magnifying  glass,  will  often  disclose  the 
faint  outlines  of  the  close-packed  rounded  grains  of  the 
original  sand.  Even  in  thin  sections  under  the  microscope, 
vein  quartz  and  quartzite  are  often  similar,  but  examination 
generally  shows  the  outlines  of  the  sand  grains  in  the 
quartzite,  marked  by  a  rim  of  clay  or  iron. 

What  is  the  nature  and  origin  of  shales? 

Mud  beds,  when  somewhat  dried,  become  clay.  On 
hardening,  clay  becomes  shale,  a  rock  distingushed  by  its 
softness,  its  fineness  of  texture,  and  its  easy  splitting  into 
thin  sheets  along  the  bedding  planes.  Shales  may  be  of 
any  color,  but  are  most  frequently  dark-colored,  often 
black. 


30  GEOLOGY  APPLIED  TO  MINING. 

What  is  a  slate? 

When  shale  becomes  still  harder,  it  is  called  slate.  The 
property  possessed  by  slates  of  splitting  into  thin  sheets, 
or  fissility,  is  usually  due  to  pressure  exerted  upon  the  rock 
subsequent  to  its  deposition.  Usually,  also,  the  splitting 
does  not  follow  the  bedding  planes,  though  sometimes  it 
may. 

How  do  limestones  and  marbles  originate? 

Limestone,  dolomite  and  marble  are  intimately  related. 
They  begin  as  the  accumulation  of  the  shells  of  marine 
animals,  often  broken  so  as  to  form  sand  or  mud;  or,  more 
rarely,  they  are  chemically  precipitated.  These  deposits 
harden  into  rocks.  Limestones  are  of  all  degrees  of  com- 
pactness, from  the  slightly  consolidated  shell-mud  to  the 
dense  semi-crystalline,  generally  dark  blue,  rock,  which  it 
is  sometimes  difficult  to  recognize  as  sedimentary.  When 
limestones  have  been  exposed  to  heat  and  pressure  in  the 
earth's  crust,  they  become  crystalline,  and  are  then  marbles. 
The  fossils  which  are  frequently  found  in  limestones  often 
become  obliterated  when  the  marble  state  is  reached,  but 
not  always. 

How  does  dolomite  originate? 

Lime-sands  and  limestones  are  generally  carbonate  of 
lime,  with  some  impurities.  Magnesia  (magnesium  oxide) 
has  a  great  affinity  for  lime  carbonate,  and  easily  combines 
with  it  to  form  dolomite,  a  mineral  containing  54.35  per 
cent  calcium  carbonate  and  45.65  per  cent  magnesium 
carbonate.  Magnesium  salts  are  present  in  most  waters, 


ARRANGEMENT    OP    STRATIFIED    ROCKS.  31 

especially  in  the  ocean  and  in  underground  waters.  Where 
sea-water  becomes  land-locked,  and  the  magnesium  and 
other  salts  concentrated,  (as  is  the  case  in  the  Dead  Sea 
and  in  Great  Salt  Lake,  for  example),  the  lime  deposits 
which  become  precipitated  are  apt  to  be  impregnated  with 
magnesium  and  to  change  to  dolomite.  Limestone  rocks 
that  are  permeated  by  underground  magnesian  waters 
may  be  similarly  altered. 

If  a  certain  dolomite  formation  is  everywhere  of  about 
the  same  composition,  the  alteration  has  probably  been 
due  to  the  first  named  cause;  if  the  dolomite  occurs  chiefly 
along  water-courses,  such  as  fissures  in  limestones,  and  is 
of  very  irregular  composition,  the  change  has  probably 
been  due  to  the  latter  agency.  Marbles  are  very  frequently 
dolomitic  or  magnesian. 

Example:  In  the  lead  and  zinc  mining  region  of  south- 
western Missouri,  there  are  beds  of  magnesian  limestone 
or  dolomite  of  Silurian  age.  This  limestone  is  magnesian 
wherever  it  outcrops  and  is  evidently  an  original  deposit. 
On  the  other  hand,  there  are  irregular  deposits  of  dolomite 
immediately  associated  with  the  ore-bodies.  This  dolo- 
mite is  generally  contiguous  to  the  limestone  wall  rocks,  and 
appears  to  grade  into  them.  Blocks  of  limestone  are  often 
found  covered  with  a  shell  of  such  dolomite,  evidently 
formed  by  the  action  of  solutions  containing  magnesia  upon 
the  limestone.* 

How  can  one  tell  limestone  from  dolomite? 

It  is  next  to  impossible  to  distinguish  dolomite  from 
limestone  by  the  appearance.  In  certain  regions  the 

*  Arthur  Winslow,  Missouri  Geological  Survey,  Vol.  VII,  p.  448. 


32  GEOLOGY   APPLIED  TO   MINING. 

dolomite  will  have  a  different  appearance  from  the  lime- 
stone, being  finer  or  coarser,  or  of  a  different  color ;  but  the 
test  will  not  hold  good  for  another  district.  The  best  way 
is  to  test  with  very  dilute  hydrochloric  acid.  A  drop  of 
this  on  limestone  causes  a  lively  effervescence,  while 
dolomite  is  slightly  or  not  at  all  attacked.  This  does  not 
apply  to  strong  acid.  A  thin  section  of  dolomite  under 
the  microscope  is  like  one  of  limestone,  but  may  often  be 
distinguished  by  the  tendency  of  dolomite  to  be  in  small 
grains  with  perfect  rhomboidal  outlines,  while  calcite  (lime 
carbonate)  is  more  frequently  a  mass  of  interlocking 
irregular  grains. 

THE  DISTINCTION  BETWEEN  BEDDING,  CLEAV- 
AGE, SCHISTOSITY,  AND  GNEISSIC  STRUCTURE. 

What  are  cleavage-planes? 

When  any  rock,  but  particularly  a  shale,  is  exposed  to 
pressure  by  the  movements  of  the  earth's  crust,  it  is  apt  to 
break  easily  into  sheets  along  certain  planes  determined 
by  the  direction  of  the  applied  forces.  These  planes  are 
called  cleavage  planes.  They  may  lie  at  any  angle  to  the 
bedding  planes,  or  may  even  coincide;  they  may  be  straight 
while  the  bedding  planes  are  folded;  in  short,  the  two  sets 
of  parting  planes  have  no  relation  to  one  another. 

How  can  one  distinguish  between  cleavage  and  stratification? 

To  be  sure  of  stratification,  one  must  look  for  layers 

differing   in  texture    or    mineral    composition.      In    the 

case  of  conglomerates    the  pebbles  typically  have  their 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  33 

longest  diameter  parallel  to  the  stratification,  for  they  have 
come  to  rest  upon  their  flat  sides,  and  not  on  end;  in  fossil- 
iferous  rocks  the  fossil  shells  have  their  long  axis  parallel 
to  the  stratification,  for  the  same  reason.*  In  short,  any 
evidence  of  the  original  position  of  the  sediments  must  be 
looked  for.  A  rock  may  not  split  at  all  along  its  true 
bedding  planes,  while  it  may  separate  perfectly  along 
cleavage  planes  which  run  across  the  stratification;  but 
the  cleavage  must  not  for  that  reason  be  mistaken  for 
bedding. 

What  are  schists  and  gneisses? 

Schists  and  gneisses  have  been  mentioned  in  Chapter  I 
as  frequently  the  result  of  the  metamorphism  of  sedi- 
mentary rocks. 

A  schist  is  a  highly  metamorphosed  slate,  which  has 
become  thoroughly  crystalline,  the  minerals  having  a 
parallel  arrangement.  The  individual  crystals  are  gener- 
ally relatively  small.  Further  crystallization  may  produce 
larger  crystals,  and  a  less  perfect  parallel  arrangement, 
when  a  gneiss  results.  Gneiss,  as  commonly  understood, 
is  composed  of  the  same  minerals  as  granite  (feldspar, 
quartz,  mica,  hornblende,  etc.)  and  differs  from  it  by  reason 
of  the  more  or  less  marked  arrangement  of  its  minerals  in 
bands.  With  variations  in  the  mineral  composition 
syenite  gneiss,  diorite  gneiss,  etc.,  are  distinguished, 


*  An  exception  to  this  test  of  stratification  planes  is  where  rocks  have  been 
stretched  by  movements  of  the  crust.  In  that  case  it  sometimes  happens  that 
pebbles,  and  even  fossils,  are  pulled  out  of  shape,  arid  so  their  long  axes  cease 
to  have  any  relation  to  the  original  stratification. 


34  GEOLOGY  APPLIED  TO  MINING. 

possessing  a  mineral  composition  similar  to  that  of  the 
igneous  rock  from  which  they  have  been  named. 

How  do  we  know  that  schists  and  gneisses  may  be  formed 
from  sedimentary  rocks? 

Not  infrequently  in  schists,  less  often  in  gneisses,  we 
may  find  evidence  of  sedimentary  origin,  in  the  shape  of 
stratification,  of  pebbles,  and  (rarely)  of  obscure  fossils. 
In  other  places  no  such  evidence  can  be  seen,  and  we 
cannot  determine  the  origin  of  the  rock,  (except  perhaps, 
by  microscopic  study),  for  schists  and  gneisses  may  also 
be  formed  from  igneous  rocks. 

What  is  schistosity  and  gneissic  structure? 

Both  schists  and  gneisses  have  strong  banding,  resembling 
stratification,  but  having  no  necessary  connection  with  it. 
In  schists,  where  the  crystals  are  well  arranged  in  parallel 
position,  the  rock  splits  very  easily  in  the  same  direction, 
especially  in  mica-schist,  where  mica  is  an  abundant 
mineral.  This  property  is  called  schistosity.  In  gneisses, 
where  the  parallelism  is  not  so  strong,  resulting  only  in  a 
more  or  less  marked  banding,  the  banding  is  called  gneissic 
structure. 

What  are  the  different  kinds  of  schists  and  gneisses ? 

According  to  the  minerals  which  they  contain,  schists 
and  gneisses  are  given  different  names.  With  schists  this 
name  is  usually  taken  from  the  predominant  mineral. 
Thus,  mica-schists  are  the  commonest;  and  we  have  also 
garnet-schists,  hornblende-schists,  etc. 


ARRANGEMENT    OP    STRATIFIED    ROCKS.  35 

DIFFERENT  GEOLOGIC  PERIODS  DURING  WHICH 
SEDIMENTARY  ROCKS  HAVE  FORMED. 

How  long  is  it  that  sedimentary  rocks  have  been  forming? 

Studies  of  geologists  have  proved  that  the  stratified 
rocks  differ  in  point  of  age — that  they  have  been  con- 
tinuously deposited  during  millions  of  years.  At  many 
places  the  sedimentary  beds  are  several  miles  in  actual 
thickness,  and  one  can  imagine  what  time  must  elapse  to 
allow  so  much  sediment  to  accumulate. 

What  proves  the  fact  of  these  great  periods  of  time? 

The  best  proof  lies  in  the  fossils  which  the  sedimentary 
rocks  contain.  By  study  of  these  remains  or  impressions 
of  animals  and  plants  a  good  idea  of  the  history  of  the 
world  has  been  obtained,  and  of  the  manner  in  which  life 
changed,  in  the  course  of  periods  compared  with  which  the 
historic  period  of  man  on  earth  is  but  as  a  day  to  a 
century. 

• 

How  did  life  begin  and  develop  on  earth? 

We  do  not  well  understand  the  beginning  of  life,  for  in 
the  oldest  rocks  the  traces  of  life  are  almost  always  de- 
stroyed by  metamorphism.  But  when  we  first  find  a  good 
record,  theie  were  already  mollusks,  crustaceans  and 
worms;  afterward  fishes  came  in,  and  then  reptiles;  still 
later  mammals,  and  finally  the  highest  type  of  mammals — 
man.  In  the  plant  world  there  was  a  like  gradual  devel- 
opment and  change. 


36  GEOLOGY   APPLIED   TO   MINING. 

How  did  the  different  geologic  periods  come  to  be  so  defined 
and  named? 

The  science  of  geology  is  new.  Within  the  last  hundred 
years,  geologists  have  studied,  in  different  parts  of  the 
world,  the  fossils  of  certain  groups  of  stratified  rocks,  and 
have  applied  names  to  the  t'.me  periods  covered  by  the 
fossils  they  have  there  found.  These  names  frequently 
come  from  the  name  of  the  country  where  the  rocks  were 
first  studied — as  Jurassic,  from  the  Jura  mountains  (part 
of  the  Alps);  Cambrian,  from  Cambria  (Wales);  etc. 
Afterward,  in  other  parts  of  the  world,  rocks  containing 
similar  fossils  were  also  called  Jurassic  or  Cambrian.  Other 
terms  came  from  some  real  or  supposed  peculiarity  of  the 
rocks  of  that  period— thus,  Carboniferous  (meaning  coal- 
bearing).  At  first  all  coal  was  supposed  to  occur  in  the 
rocks  of  this  age ;  and  conversely,  all  rocks  of  this  age  were 
expected  to  contain  coal.  More  recently  both  propositions 
have  been  completely  disproved,  but  the  name  remains. 
Other  names  are  left  to  us  from  earlier  and  cruder  attempts 
at  age  classification.  Thus  Tertiary  and  Quaternary 
remain  from  a  division  of  rocks  into  primary,  secondary, 
tertiary  and  quaternary.  The  first  two  of  these  terms  have 
been  dropped,  the  last  two  retained. 

Does  the  classification  of  geologic  time  by  periods  represent  a 
natural  system? 

The  classification  is  more  or  less  arbitrary  and  might  be 
just  as  accurate  if  it  were  made  up  quite  differently  from 
what  it  is.  Between  one  period  and  another  we  must  not 


ARRANGEMENT    OF    STRATIFIED    ROCKS. 


37 


imagine  that  there  were  sharp  divisions.  Life  and  the 
deposition  of  sediments  often  passed  smoothly  and  unin- 
terruptedly from  one  period  into  another.  However, 
the  classification  is  of  accepted  usage  and  enables  general 
understanding. 

What  are  the  names  of  the  geologic  periods  of  time? 

Following  are  the  chief  divisions,  beginning  with  the 
oldest : 


Archaean 


Paleozoic 


Mesozoic 


Cenozoic 


What  are  the  characteristics  of  rocks  of  the  Archcean  period '? 

The  Archaean  rocks  contain  no  fossils  and  show  no  signs 
of  life.  Although  it  is  probable  that  life  existed  at  that 
period  or  during  a  portion  of  it,  the  traces  have  been 
obliterated.  The  Archaean  rocks  are  usually  metamorphic. 
They  may  be  gneisses  and  schists,  or  massive  crystalline 
rocks,  like  granites  They  occupy  large  areas  of  the 
earth's  surface. 


)  No  fossils.     No 
)      known  life. 

Cambrian 

Silurian 

Devonian 

Carboniferous 

Each   marked  by 

Triassic 

>•     characteristic 

Jurassic 

fossils. 

.Cretaceous 

[Tertiary 

[  Quaternary 

38  GEOLOGY  APPLIED  TO  MINING. 

What  are  the  characteristics  of  rocks  of  the  Cambrian  period? 

The  Cambrian  rocks  contain  trilobites,1  (often  large), 
certain  generally  very  small  brachiopods,  worm-tracks, 
some  remains  of  fossil  sponges,  crinoids,  etc.;  also  traces 
of  seaweeds. 

What  are  the  characteristics  of  rocks  of  the  Silurian  period? 

The  Silurian  rocks  contain  impressions  of  sea- weeds; 
some  terrestrial  plants,  among  them  some  members 
of  the  Lepidodendron2  family,  having  much  the  habit  of 
the  spruce  or  pine  tribe;  trilobites3  and  many  other  crus- 
taceans;4 worms;  very  many  graptolites5  (feather-like 
animals);  many  mollusks,6  especially  brachiopods7  and 
cephalopods  ;8  also  many  lamellibranchs,9  corals  and 
crinoids.10  The  earliest  fishes  also  occur,  some  of  them  of 
the  shark  tribe. 

What  are  the  characteristics  of  rocks  of  the  Devonian  period? 
The  Devonian  rocks  contain  sea- weeds,  lycopods11 
(ground  pines)  and  ferns,  equisetse12  or  horse-tails,  and 
conifers.13  The  animal  life  was  varied.  There  were  many 
sponges  and  corals,  crinoids,  brachiopods,  and  other 
kinds  of  mollusks,  and  a  few  trilobites.  .Fish  are  fre- 
quently -found,  belonging  to  the  shark  and  other  tribes. 
The  Devonian  has  been  called  the  Age  of  Fishes. 


1  For  the  explanation  of  this  and  following  terms,  see  pp.  44-51.  2See  p.  51. 
•  See  p.  47.  4  See  p.  47.  5  See  p.  45.  •  See  p.  45.  7  See  p.  47.  8  See  p.  46. 
9  See  p.  46.  "  See  p.  45.  »  See  p.  51.  »  See  p.  51.  13  See  p.  50. 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  39 

What  are  the  characteristics  of  rocks  of  the  Carboniferous 

period? 

The  Carboniferous  was  marked  by  an  abundance  of 
vegetation,  whose  remains  or  imprints  we  often  find  as  coal 
or  as  fossils  in  the  rocks.  There  were  lepidodendron  and 
sigillaria,1  and  various  ferns,  conifers,  and  calamites,2  (a 
genus  of  horse-tails) .  As  to  the  animal  life,  there  was  a 
great  abundance  of  crinoids  or  sea-lilies;  also  numerous 
brachiopods,  cephalopods,3  etc.  Besides  fishes,  remains  of 
amphibians  occur,  some  snake-like,  some  lizard-like,  some 
frog-like.  On  the  land  were  insects,  (cockroaches,  etc.) 
spiders  and  centipedes,  and  true  reptiles — snakes,  saurians/ 
and  turtles. 

What  are  the  characteristics  of  rocks  of  Mesozoic  age? 

The  Triassic,  the  Jurassic  and  the  Cretaceous  periods 
together  constitute  the  Mesozoic  age,  called  the  Age  of 
Reptiles.  In  this  age  came  the  first  mammals,  the  first  of 
the  common  or  osseous6  fishes,  the  first  palms  and  angio- 
sperms.7 

What  are  the  characteristics  of  rocks  of  the  Triassic  period? 

The  Triassic  had  neither  the  sigillarids  nor  the  lepido- 
dendrids  of  the  Carboniferous  era;  but  many  cycads, 
besides  ferns,  horse-tails,  and  conifers.  As  to  animals, 
some  brachiopods  and  lamellibranchs  were  abundant; 
also  ammonites.9  Fishes  and  reptiles,  the  latter  including 
the  gigantic  dinosaur,10  were  also  plentiful. 

1  See  p.  51.  2  See  p.  51  3  See  p.  46.  *  See  p.  48.  6  See  p.  49.  •  See  p.  49. 
7  See  p.  50.  8  See  p.  50.  »  See  p.  47.  "Seep.  49. 


40  GEOLOGY  APPLIED  TO  MINING. 

What  are  the  characteristics  of  rock  of  the  Jurassic  period? 

The  Jurassic  contains,  besides  many  characteristic 
radiates/  sponges  and  mollusks,  (brachiopods,  lamelli- 
branchs,  cephalopods,  etc.)  remains  of  gigantic  reptiles, 
including  the  flying-reptiles  or  pterodactyls,2  the  ichthy- 
osaurus,3 tortoises,  etc.;  also  some  mammals.  Fishes 
flourished. 

What  are  the  characteristics  of  rocks  of  the  Cretaceous  period? 
The  Cretaceous  plants  were  marked  by  the  first  great 
development  of  the  angiosperms  (including  all  plants  with 
a  bark,  except  the  conifers  and  cycads).  This  class 
embraces  the  oak,  willow,  maple,  etc.  The  smaller  marine 
animals  have  contributed  shells  in  great  variety  and 
profusion  to  the  Cretaceous  sediments.  Rhizopods  with 
tiny  shells  (f oraminif ers") ,  were  abundant,  and  constitute 
most  of  the  chalk  beds.  Sponges  and  corals  were  of  great 
importance.  The  oyster  family  flourished,  and  many 
others.  Sharks  and  other  fishes  were  common;  reptiles 
were  numerous,  among  them  true  sea-serpents,  as  much 
as  seventy-five  feet  long.  Turtles  lived,  and  also  birds, 
some  of  which  possessed  pointed  teeth. 

What  are  the  characteristics  of  rocks  of  the  Tertiary  period? 

The  Tertiary  is  called  the  Age  of  Mammals,  for  during 
this  period  mammals  flourished.  Yet  most  of  the  Tertiary 
mammal  species  are  now  extinct. 

The  Tertiary  beds  often  contain  plant  remains,  belonging 

1  See  p.  45.     2  See  p.  49.     3  See  p.  49.     4  See  p.  44. 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  41 

to  species  of  oak,  maple,  dog-wood,  magnolia,  fig,  palm,  etc. 
The  mollusks  comprise  many  species  of  oyster,  clam;  and 
other  lamellibranchs,  but  few  brachiopods.  Crabs,  insects, 
fishes,  etc.  were  plentiful.  Crocodiles,  snakes,  and  turtles 
abounded.  There  were  many  large  mammals,  including 
now  extinct  species  of  elephant,  tapir,  rhinoceros,  horse, 
tiger,  lion,  wolf,  peccary,  etc.  The  remains  of  all  of  these 
are  found  in  the  western  United  States.  In  the  sea  there 
were  whales,  dolphins,  seals  and  walruses. 

Example:  A  quarry  near  Carson,  Nevada,  now  used  as  a 
State  prison  yard,  is  cut  in  grayish  sandstones,  whose 
bedding  is  such  as  to  indicate  that  they  were  deposited  at 
the  mouth  of  an  ancient  stream.  When  the  sandstone  was 
removed  down  to  two  shale  layers,  these  were  found  liter- 
a'ly  covered  with  the  tracks  of  many  species  of  birds  and 
mammals,  including  the  mammoth,  the  deer,  the  wolf, 
many  birds,  a  horse,  and  tracks  resemb'ing  those  of  a  man, 
but  which  may  have  been  made  by  some  animal.  The 
footprints  and  some  associated  bones  indicate  a  probable 
very  late  Tertiary  age.  These  remarkable  tracks  were 
exposed  and  trampled  over  by  horses  and  men  for  eight  or 
ten  years,  without  attracting  any  especial  attention.* 

What  are  the  characteristics  of  rocks  of  the  Quaternary  period? 
The  Quaternary  in  called  the  Age  of  Man,  for  in  this 
period  the  human  race  began  to  flourish.  During  this  age 
came  the  Glacial  Period  or  Ice  Age,  when  a  vast  glacier  or 
glaciers  covered  the  northern  part  of  North  America,  with 
their  southernmargin  running  irregularly  across  the  northern 
and  central  part  of  the  present  United  States.  These 

*  J.  Le  Conte,  Proceedings  California  Academy  of  Science,  Aug.  27,  1882. 


42  GEOLOGY  APPLIED  TO  MINING. 

glaciers  spared  most  of  Alaska  and  some  other  areas,  but 
everywhere  else  ground  off  the  cliffs  and  hills,  and  left, 
on  melting,  vast  deposits  of  boulders  and  gravels.  Thus  in 
the  eastern  United  States  the  southern  and  central  por- 
tions, which  are  unglaciated,  are  strikingly  different  from 
the  rocky,  bouldery,  often  barren  glaciated  areas  of  the 
north.  In  the  Western  States  the  glacier  did  not  extend 
far  south  of  the  present  Canadian  boundary. 

In  the  early  part  of  the  Quaternary  flourished  many 
great  mammals,  of  species  now  generally  extinct.  These 
comprised  gigantic  mastodons,  elephants  (mammoths), 
lions,  hyenas,  bears,  wolves,  beavers,  etc.  Man  probably 
lived  in  the  later  Tertiary  period,  but  evidence  of  his 
existence  is  first  complete  in  the  Quaternary.  He  was 
contemporary  with  the  hairy  mammoth  and  other  extinct 
species,  which  he  has  survived. 

How  can  one  tell  to  what  period  a  given  rock  belongs? 

The  only  reliable  way  to  identify  a  stratified  rock,  as 
belonging  to  one  of  these  periods,  is  by  study  of  the  fossil 
remains  which  it  contains.  These  will  tell  in  what  stage 
of  the  world's  history  the  sediment  was  laid  down.  To 
the  paleontologist,  who  has  made  a  careful  study  of  extinct 
animal  forms,  it  is  generally  possible,  on  seeing  a  group  of 
fossils,  to  refer  them  to  one  of  the  great  periods  given. 
But  to  the  casual  observer,  the  differences  are  not  so 
striking  as  to  be  retained  without  study  and  careful  com- 
parison. There  are,  to  be  sure,  certain  broad  signs,  which 
he  may  use  as  guides,  to  a  limited  extent.  Rocks  con- 
taining large  numbers  of  trilobites,  with  few  other  fossils. 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  43 

are  probably  Cambrian,  or  Silurian,  most  probably  the 
former.  Rocks  with  graptolites  are  probably  Silurian. 
Rocks  containing  plant  remains,  if  these  are  largely  reed- 
like  and  otherwise  unlike  any  of  our  common  trees,  are 
probably  Carboniferous,  perhaps  Devonian.  If  the  plant 
remains  consist  of  leaves  resembling  those  of  our  modern 
trees,  the  rocks  are  probably  Cretaceous  or  Tertiary.  In 
the  same  way,  rocks  containing  fossil  shells  almost  exactly 
like  those  which  are  now  occupied  by  living  animals  on  the 
sea-shore,  are  probably  Tertiary,  and  in  proportion  as  the 
unlikeness  increases,  we  can  suspect  an  older  age.  A  great 
abundance  of  crinoids  suggests  the  Carboniferous.  Abun- 
dance of  sponges,  corals  and  large  brachiopods,  with 
fewer  lamellibranchs,  suggest  the  Carboniferous  or  Devo- 
nian. Predominance  of  lamellibranchs  indicates  a  probable 
age  not  older  than  Triassic,  etc. 

With  a  little  practice  in  observing  rocks  of  known 
age,  one  can  tell  ordinarily,  although  by  no  means  always,  a 
Paleozic  rock,  a  Mesozoic  rock,  or  a  Tertiary  rock,  from  the 
general  look  of  the  fossils,  even  if  one  cannot  determine  a 
single  species.  Where  a  certain  series  of  strata  has  been 
determined  by  paleontologists,  one  can  attentively  examine 
the  fossils  contained,  and  can  then  very  likely  recognize 
beds  of  the  same  age  in  another  part  of  the  same  district 
or  even  in  another  district. 

CHARACTERISTICS  OF  THE  DIFFERENT  FOSSILS.* 
The  fossils  which  have  been  referred  to  belong  mostly  to 

*  The  descriptions  and  definitions  under  this  head  are  adopted  directly  from 
Dana's  '  Manual  of  Geology.' 


44  GEOLOGY  APPLIED  TO  MINING. 

the  anima1  kingdom ;  some  of  them  to  the  vegetable  king- 
dom. 

How  is  the  animal  kingdom  divided? 

The  animal  kingdom  is  divided  into  five  sub-kingdoms. 
Beginning  with  the  lowest  they  are: 

1.  Protozoans. 

2.  Radiates. 

3.  Mollusks. 

4.  Articulates. 

5.  Vertebrates. 

What  are  protozoans? 

Protozoans  are  minute  animals,  (usually  from  a  100th 
to  a  10,000th  of  an  inch  in  length.) 

They  have  no  external  organs  save  a  mouth  and  minute 
thread-like  organs,  and  no  digestive  apparatus  beyond  a 
stomach.  The  stomach  and  the  mouth  are  sometimes 
wanting.  There  is  no  heart  or  circulating  system  beyond 
a  palpitating  vesicle. 

What  are  rhizopods  and  foraminifers? 

Among  protozoans,  the  rhizopods  are  of  especial  interest. 
The  shells  are  usually  much  smaller  than  the  head  of  a  pin. 
The  most  common  kinds  have  calcareous  shells  called 
foraminifers,  and  these  have  contributed  largely  to  the 
formation  of  the  limestone  strata.  They  consist  of  one  or 
more  shells,  and  the  compound  kinds  present  various 
shapes. 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  45 

What  are  radia'esf 

Radiates  have  a  radiate  structure,  like  a  flower — that  is, 
they  have  similar  parts  or  organs  repeated  around  a  vertical 
axis.  These  animals  have  a  mouth  and  stomach  for  eating 
and  digestion,  and  are  widely  diverse  from  plants,  although 
resembling  them  in  their  radiate  arrangement  of  parts. 

What  are  crinoids? 

Among  the  radiates,  crino:ds  are  animals  like  an  'nverted 
star-fish  or  sea-urchin,  standing  on  a  stem  like  a  flower. 

What  are  graptolites? 

The  graptolites  were  ancient,  delicate  plume-like  animals 
which  belonged  in  the  sub-kingdom  of  radiates. 

What  are  mol  usksf 

Mollusks  possess  a  soft  fleshy  bag,  contain:ng  the  stomach 
and  viscera.  They  do  not  possess  a  radiate  structure  nor 
jointed  appendages.  Similar  parts  are  repeated  on  right 
and  left  sides  of  a  median  plane,  and  not  around  a  vertical 
axis,  as  in  radiates. 

How  are  mollusks  subdivided? 
Mollusks  are  divided  mto: 

1.  Ordinary  mollusks. 

2.  Ascidian  mollusks. 

3.  Brachiate  mollusks. 

The  ordinary  mollusks  are  divided  into: 

1.  The  acephals,  or  headless  mollusks,  the  head  not 


46  GEOLOGY  APPLIED  TO  MINING. 

being  distinctly  defined  in  outline;  as  the  oyster 
and  clam. 

2.  The  cephalates,  having  a  denned  head;  as  the  snail. 

3.  The  cephalopods,  having  the  head  furnished  with 

long  arms  (or  feet) ;  as  the  cuttle-fish. 

What  are  lamellibranchsf 

The  acephals,  or  headless  mollusks,  are  illustrated  in  the 
group  of  lamellibranchs.  These  common  species  are  well 
known  as  bivalves.  One  valve  is  on  the  right,  and  the 
other  on  the  left,  of  the  animal.  The  clam  and  the  oyster 
are  familiar  examples. 

What  are  gasteropodsf 

in  the  cephalates,  one  of  the  two  groups  are  the  gastr.ro- 
pods.  These  are  contained  in  univalve  shells  (shells  all  in 
one  piece).  The  animal  crawls  on  a  flat  spreading  fleshy 
organ  called  the  foot.  The  snail  is  a  familiar  example. 

What  are  cephalopods? 

Cephalopods,  or  cuttle-fishes,  are  of  two  kinds,  one 
having  external  shells  and  four  gills;  and  another  having 
sometimes  internal  shells,  but  not  external,  and  having 
but  two  gills.  The  external  shells  are  distinguished  from 
those  of  the  gasteropods  or  ordinary  univalves  by  nearly 
always  having  transverse  partitions — whence  they  are 
called  chambered  shells.  They  may  be  straight  or  coiled, 
but  when  coiled  are  usually  coiled  in  a  plane,  and  not  a 
spiral.  The  animal  occupies  the  outer  chamber  of  the 
shell.  The  nautilus  is  an  example. 


ARRANGEMENT    OP    STRATIFIED    ROCKS.  47 

Modern  cephalopods  are  almost  exclusively  naked 
species,  such  as  the  cuttle-fish  and  squid. 

What  are  ammonites? 

The  ammonites  are  a  genus  of  cephalopods,  and  have 
very  beautiful  and  often  large  coiled  and  fluted  shells. 

What  are  brachiopods? 

Among  brachiate  mollusks,  brachiopods  have  a  bivalve 
shell,  and  in  this  respect  are  like  ordinary  bivalves.  But 
the  shell,  instead  of  covering  the  right  and  left  sides,  covers 
the  dorsal  and  ventral  sides,  or  its  plane  is  at  right  angles 
to  that  of  a  clam.  Moreover,  it  is  symmetrical  in  form, 
and  equal  on  either  side  of  a  vertical  line.  The  valves  are 
almost  always  unequal;  the  larger  is  the  ventral,  and  the 
smaller  the  dorsal. 

What  are  articulates? 

Articulates  consist  of  a  series  of  joints  or  segments.  The 
legs,  where  any  exist,  are  jointed,  and  there  is  no 
internal  skeleton.  The  articulates  include  worms,  crusta- 
ceans, and  insects. 

What  are  crustaceans? 

The  crustaceans  have  the  body  in  two  parts — the  front 
consisting  of  a  head  and  thorax,  the  hinder  part  of  the 
abdomen.  Crabs,  lobsters,  and  shrimps  are  examples.  . 

What  are  trilobites? 

Among  crustaceans,  the  trilobites  existed  only  in  Paleo- 
zoic time.  They  had  jointed  bodies  with  a  crust-like 


48  GEOLOGY   APPLIED   TO   MINING. 

exterior.     They  are  more  like  the  horse-shoe  crab  of  the 
Atlantic  coast  than  any  other  living  species. 

What  are  vertebrates? 

The  fifth  animal  sub-kingdom  comprises  the  vertebrates. 
These  have  a  jointed  internal  skeleton,  and  a  bone-sheathed 
cavity  along  the  back  for  the  great  nervous  cord,  distinct 
from  the  cavity  of  the  viscera. 

How  are  vertebrates  divided? 
The  classes  of  vertebrates  are: 

1.  Mammals,  which  are  the  highest  branch  of  the  animal 
kingdom.     They   suckle   their   young   and   breathe   with 
lungs.     Ordinary  quadrupeds  (four-footed  animals)  are  all 
mammals. 

2.  Birds  produce  their  young  in  the  egg  form.    They  have 
a  heart  of  four  cavities;  they  breathe  by  lungs,  are  covered 
with  feathers,  and  are  adapted  for*  flying. 

3.  Reptiles  produce  their  young  in  the  egg  form.    They 
breath  by  lungs,  have  a  heart  of  three  or  four  cavities;  and 
are  naked  or  covered  with  scales.     Examples  are  crocodiles, 
lizards,  turtles,  and  snakes. 

4.  Amphibians  produce  their  young  in  the  egg  form. 
When  young  they  breath  by  gills,  and  afterward  by  lungs 
alone.     They  possess  a  heart  with  three  cavities,  and  are 
naked  or  covered  with  scales.     Examples  are  frogs  and 
salamanders. 

5.  Fishes  usually  produce  their  young  in  the  egg  form. 
They  possess  a  heart,  usually  of  two  cavities.    They  breathe 
by  gills,  and  are  naked,  or  covered  by  scales. 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  49 

What  are  osseous  fishes? 

The  osseous  fishes  or  teliosts  include  nearly  all  modern 
kinds,  except  the  sharks  and  rays.  They  usually  have 
membranous  scales.  They  are  not  known  among  fossils 
before  the  Middle  Mesozoic. 

How  are  reptiles  classified? 

Reptiles  are  divided  into  snakes,  saurians,  and  turtles. 
The  saurians  vary  in  length  from  a  few  inches  to  fifty  feet. 

What  were  dinosaurs  and  pterosaurs? 

Among  the  saurians  the  tribe  of  dinosaurs,  reptiles  of 
great  size,  now  extinct,  possessed  some  mammalian  and 
many  bird-like  characteristics. 

Another  group  were  the  pterosaurs  or  the  flying  saurians. 
The  pterodactyls  were  the  most  common  genus.  The  little 
finger  of  the  forefoot  was  excessively  prolonged,  and  from 
this  a  membrane  extended  to  the  tail  and  made  a  wing  for 
flying.  The  remaining  fingers  were  short,  and  furnished 
with  claws.  They  had  the  habits  of  bats,  and  wings  of  a 
s'milar  character. 

What  were  ichthyosaurs? 

The  ichthyosaurs  were  a  genus  of  the  group  of  enalio- 
saurs,  or  swimming  saurians.  They  were  gigantic  animals, 
10  to  40  feet  long,  having  paddles  somewhat  like  the 
whale,  long  head  and  jaws,  and  an  eye  of  enormous 
dimensions. 


50  GEOLOGY  APPLIED  TO  MINING. 

How  is  the  vegetable  kingdom  divided? 

The  vegetable  kingdom  is  primarily  divided  into  crypto- 
gams, which  have  no  distinct  flowers  or  proper  fruit,  (such 
as  ferns  and  sea- weed),  and  phenogams,  having  distinct 
flowers  and  seed,  (such  as  our  ordinary  trees  and  plants). 

How  are  the  phenogams  subdivided? 
The  phenogams  are  divided  into : 

1.  Gymnosperms.     The  plant  has  a  bark,  and  grows  by 
an  annual  addition  to  the  exterior  of  the  wood,  thus  forming 
rings  of  growth.    The  flowers  are  very  simple,  and  the  seed 
naked.     Examples   are   the   pine,   spruce,   hemlock,    etc. 
Gymnosperms  include:  (1)    Conifers.      (2)  Cycads.     The 
conifers   include  most  of   the   common   evergreen  trees. 
Their  wood  is  simply  woody  fibre,  without  ducts.     The 
cycads  had  the  habit  of  palms,  while  related  to  the  pine 
tribe. 

2.  Angiosperms.     Growing   by   external    annual    rings, 
like  the  gymnosperms;  having  regular  flowers  and  covered 
seeds.     Examples  are  the  maple,  elm,  rose,  etc. 

3.  Endogens,  having  regular  flowers  and  seed,  but  with 
no  bark  and  no  rings  of  growth.     In  a  transverse  section 
of  a  trunk  or  stem,  the  ends  of  fibres  are  shown.     Examples 
are  the  palm,  Indian  corn,  lily,  etc. 

How  are  the  cryptogams  subdivided? 
The  cryptogams  are  subdivided  into: 
1.  Thallogens.     Consisting  wholly  of  cellular  tissue; 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  51 

growing  mostly  in  fronds  without  stems,  and  in  other 
spreading  forms;  as,  (1)  Algse,  or  sea- weeds.  (2)  Lichens. 
(3)  Fungi,  or  mushrooms. 

2.  Anogens.    Consisting  wholly  of  cellular  tissue;  grow- 
ing up  in  short,  leafy  stems;  as,  (1)  Mosses.      (2)    Liver- 
worts. 

3.  Acrogens.    Consisting  of  vascular  tissue  in  part,  and 
growing  upward;  as,  (1)    Ferns.      (2)    Lycopods    (ground 
pine).   (3)    Equiseta,  or  horse-tail;  and  including    many 
trees  of  the  coal  period. 

What  were  lepidodendrids? 

Among  the  fossil  lycopods,  the  lepidodendrids  (tall  trees, 
with  the  exterior  embossed  with  scars  in  alternate  order,) 
were  of  many  kinds.  In  foliage,  they  resembled  the  pines 
and  spruces  of  the  present  day. 


What  were  sigillarids? 

The  sigillarids  differed  from  the  lepidodendrids  in  having 
the  scars  in  vertical  order.  The  trunk  was  woody,  but  not 
firm  within;  and  it  had  a  large  pith. 

What  were  calamitesf 

In  the  Paleozoic,  the  equisetse,  or  horse-tails,  were  re- 
presented by  plants  called  calamites.  They  had  a  reed- 
like  appearance,  with  jointed  stem,  and  finely  furrowed 
surface. 


52  GEOLOGY  APPLIED  TO  MINING. 

THE  ORDER  OF  SUCCESSION  AS  FOUND  IN  ACTUAL 
PRACTICE. 

7s  the  succession  of  geologic  periods,  as  previously  stated, 
always  shown  in  the  sedimentary  rocks  of  a  given  district? 
Not  always ;  indeed,  not  usually.     Any  one  of  the  geologic 
ages  may  not  be  represented  at  all,  may  be  represented  by 
very  thin  beds,  or  by  strata  thousands  of  feet  thick.    Often 
rocks  of  only  a  few  of  the  geologic  ages  are  present.    Ter- 
tiary beds  may  rest  directly  upon  those  of  the  Cambrian 
age;  or  the  Quaternary  may  rest  upon  the  Archaean.     In 
fact,  there  is  every  possible  combination. 

What  is  the  reason  for  this  lack  of  uniformity? 

In  a  certain  part  of  the  earth's  surface  there  may  have 
been  no  sediments  during  a  certain  age  or  ages.  If  the 
region  was  a  land  area  during  any  period  this  would  be  the 
case,  for  stratified  rocks  are  laid  down  in  water.  Again, 
after  the  sediments  were  formed,  during  a  certain  period, 
they  may  have  changed  into  land,  and  have  been  stripped 
off  and  carried  away  by  the  erosion  of  the  rivers.  Then, 
when  the  land  became  again  submerged  and  new  strata 
were  deposited  in  their  place,  the  new  beds  may  have  come 
to  rest  on  those  of  an  age  much  greater  than  the  series 
usually  found  next  underneath  them. 

What  is  an  erosion  gap  and  an  unconformity f 

In  the  case  just  mentioned,  the  line  of  contact  between 
the  two  series  of  strata  will  be  irregular,  for  the  newer  beds 
will  have  been  deposited  over  the  hills  and  hollows  of  the 
older  topography,  although  the  stratification  in  both 


ARRANGEMENT    OF    STRATIFIED    ROCKS. 


53 


series  may  be  parallel.      This    is  called   an   erosion  gap; 
sometimes  an  unconformity  by  erosion. 

Frequently,  in  the  interval  between  the  deposition  of  the 
earlier  series  and  that  of  the  later  one,  there  occur  move- 
ments in  the  earth's  crust,  so  that  the  beds  of  the  older 
series  are  bent  or  folded;  then  the  new  series  does  not  have 
its  stratification  parallel,  but  rests  discordantly  on  the  bent 
and  worn  edges  of  the  old  strata.  This  is  a  true  uncon- 
formity (Fig.  2). 


Fig.  2.  Ideal  sketch  to  illustrate  unconformities,     a.  Earlier  line  of  unconform- 
ity; b.  Later  line. 

RELATION  OF  PHYSICAL  CHARACTERS  TO  GEOLOGIC  AGE. 
Can  one  tell  from  the  kind  of  rock  what  age  it  belongs  to? 

In  general,  the  nature  of  the  strata,  whether  limestone, 
quartzite,  etc.,  is  of  little  value  in  determining  to  which  of 
the  geological  periods  it  belongs.  A  certain  stage  of  the 
Carboniferous,  for  example,  may  be  represented  by  a 
sandstone  in  one  district,  a  limestone  in  another. 

Is  a  given  bed  necessarily  of  one  kind  of  rock  throughout? 

A  bed  may  pass  laterally  from  one  kind  of  rock  into 
another  (as  from  a  sandstone  into  a  limestone),  within  a 
space  of  a  few  miles  or  even  much  less. 


54  GEOLOGY  APPLIED  TO  MINING. 

Example:  On  the  Kuskokwim  river,  Alaska,  the  writer* 
observed  a  series  of  interbedded  sandstones  and  shales, 
where  the  sandstone  passes  laterally  into  shale  in  a  remark- 
able way.  A  thick  bed  of  sandstone  splits  into  separated 
beds  which  become  alternate  with  beds  of  shale,  and  these 
rapidly  grow  thinner  as  the  shale  beds  increase  in  thickness 
until  the  shale  forms  nearly  the  entire  rock  (Fig.  3). 


Fig.  3.  Sketch  of  cliff  on  Kuskokwim  river,  Alaska,  showing  sandstones  and 
shales  passing  laterally  into  one  another. 


What  is  the  explanation  for  this  transition? 

If  one  considers  sediments  now  being  formed,  one  will 
find  such  transitions  common  along  the  sea-shore.  For 
example,  there  is  found  at  one  point  mud,  at  another  sand, 
at  another  gravel  or  cobbles,  all  being  deposited  at  the  same 
time.  When  such  a  deposit  is  hardened  and  becomes 
stone,  it  will  show  within  a  single  bed  the  change  from  a 
shale  to  a  sandstone,  and  from  a  sandstone  into  a  con- 
glomerate. 

*  20th  Annual  Report  United  States  Geological  Survey,  Part  VII,  p.  125. 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  55 

Can  strata  be  identified  by  their  physical  characters? 

If  only  the  foregoing  exception  is  remembered  as  a  guard 
against  over-confidence,  it  is  possible  to  trace  certain  strata 
a  long  distance  by  their  physical  characters,  which  they  may 
retain  for  hundreds  of  miles.  Sometimes  the  peculiarities 
of  certain  strata  are  so  marked  that,  when  one  finds  in 
adjacent  districts  rocks  possessing  these  peculiarities,  there 
is  a  strong  suggestion  of  identity  of  age. 

Example:  The  lithological  identity  of  the  peculiar  parti- 
colored, thin-bedded  shales,  lithographic  limestones  and 
quartzite  of  the  Parting  Quartzite  series  in  Aspen,  Colorado, 
with  Devonian  strata  described  by  Mr.  C.  D.  Walcott  from 
Kanab  creek,  Utah,  so  impressed  the  writer,  that  other 
conditions  being  favorable,  he  provisionally  classified  the 
Aspen  beds  as  Devonian,  although  the  localities  are 
hundreds  of  miles  apart.  This  correlation  was  afterward 
borne  out  by  the  discovery  of  Devonian  fossils  in  the 
Aspen  beds. 

Yet  correlation  from  physical  characters  alone,  without 
sufficient  guardedness  and  auxiliary  evidence,  has  fre- 
quently led  into  the  most  awkward  errors. 

Does  a  certain  kind  of  rock  ever  constitute  beds  of  the  same  age 
over  large  areas? 

A  certain  character  of  strata  may  persist  in  some  in- 
stances over  a  wide  region — even  over  large  portions  of 
continents.  Some  physical  features  of  strata  of  a  certain 
period  seem  to.  be  of  almost  world- wide  occurrence.  In 
rocks  belonging  to  the  Triassic,  all  over  the  world,  there  is 


56  GEOLOGY  APPLIED  TO  MINING. 

an  astonishing  quantity  of  massive  red  sandstone;  yet  all 
massive  red  standstones  are  by  no  means  Triassic,  and 
conversely,  all  Triassic  rocks  are  not  red  sandstone. 

COMPARISON  AND  CORRELATION. 
Mode  of  Determining  the  Relative  Age  of  Different  Strata. 

How  can  one  determine  the  relative  age  of  different  series  of 

strata  in  contact  with  one  another? 

The  relative  age  of  a  series  of  strata  may  often  be  deter- 
mined by  making  reference  to  other  strata,  the  age  of  which 
is  definitely  known.  The  first  rule  to  be  observed,  is  the 
simple  rule  of  superposition,  by  which  the  upper  of  two 
series  of  strata,  or  of  two  beds,  is  usually  the  younger.  This 
is  to  be  applied  not  only  where  the  beds  are  horizontal  but 
where  they  are  folded;  it  is  easy  to  do  this,  except  in  the 
rare  case  where  the  folding  has  been  so  great  that  some  beds 
have  been  overturned,  and  the  normally  lower  ones  come 
on  top.  In  this  case  the  study  of  the  folding,  the  fossils 
present  in  the  rock,  etc.,  will  afford  the  data  requisite  for 
solving  the  problem. 

This  rule  does  not  necessarily  apply  where  a  fault 
separates  the  two  beds  in  question. 

How  may  the  relative  age  of  different  series  of  strata,  not  in 

contact  with  one  another,  be  judged? 

Where  two  rocks  are  not  found  in  actual  contact,  there 
are  other  tests  of  their  relative  age.  Of  two  rocks  close 
together,  the  more  metamorphosed  or  hardened  one  is 
probably  the  older;  likewise,  that  one  which  shows  most 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  57 

folding  and  faulting,  and  other  evidence  of  disturbance,  is 
probably  the  older.  One  rock,  such  as  conglomerate,  may 
contain  pebbles  which  have  been  derived  from  the  other: 
in  this  case  the  conglomerate  is  clearly  the  younger.  Many 
such  tests  will  present  themselves  to  the  careful  inves- 
tigator. 

Mode  of  Correlating  Similar  Strata  in  Adjacent  or  Separated 

Regions. 

How  are  similar  strata  in  different  localities  identified  as 
such? 

This  point  has  already  been  dwelt  upon,  but  may  be 
briefly  summarized. 

The  correlation  or  matching  of  similar  strata  in  adjacent 
or  separated  regions  is  usually  done  by  means  of  the  fossils 
which  they  contain.  If  these  fossils  are  nearly  the  same  in 
two  different  localities,  the  beds  may  be  correlated;  or,  if, 
although  not  belonging  to  the  same  species,  they  are  known 
to  represent  a  similar  stage  in  the  development  of  life. 

When  fossils  are  lacking,  the  physical  characteristics 
may  serve  as  a  basis  for  correlation.  The  bed  in  question 
may  be  traced  wholly  or  partly  from  one  district  to  another; 
or,  if  its  peculiarities  are  very  striking,  a  correlation 
between  separated  districts  may  often  be  made  without 
tracing  out  the  intervening  portions.  Thin  beds  are 
usually  traced  out  and  correlated  by  their  physical  charac- 
teristics, which  are  apt  to  be  more,  definitive  in  this  case 
than  fossils, 


58  GEOLOGY  APPLIED  TO  MINING. 

THE    ASSOCIATION    OF    VALUABLE    MINERALS 
WITH  CERTAIN  STRATA. 

What  is  the  commercial  application  of  the  knowledge  of  the 
principles  concerning  strata? 

Whenever  valuable  minerals  are  confined  wholly  or 
partly  to  a  certain  bed  or  beds,  the  ability  to  recognize  and 
trace  that  bed  in  different  localities  often  leads  to  the 
discovery  of  new  mineral  districts  and  of  new  mines. 

GENERAL  RELATIONS  OF  STRATIFIED  ORES. 

How  is  it  that  minerals  are  sometimes  preferentially  associated 

with  a  certain  sedimentary  bed  or  beds? 

The  association  of  a  valuable  mineral  with  certain  sedi- 
mentary beds  may  be  either  primary  or  subsequent.  That 
is,  the  mineral  may  have  originally  been  precipitated  in 
bed  form,  along  with  the  other  strata,  or  it  may  have  been 
introduced  into  the  beds  long  after  their  deposition. 

Among  valuable  minerals  known  to  be  deposited 
originally  in  bed  form,  we  may  cite,  besides  coal, 
some  gypsum  deposits,  salt  beds  and  many  other 
non-metallic  minerals.  In  this  way  metallic  minerals  are 
also  deposited  in  a  greater  or  less  state  of  concentration. 
Iron  and  manganese  are  deposited  both  in  bogs  and  in  the 
ocean  depths;  and  in  some  rich  muds  copper  in  slight 
amounts  and,  to  a  certain  extent,  even  silver  and  gold  are 
precipitated.  Many  of  these  first  named  deposits  afford 
workable  minerals  just  as  they  are  deposited;  others, 
especially  those  of  the  less  common  metals,  require  further 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  59 

concentration  by  percolating  waters,  but  in  the  end  the  ore 
will  still  be  confined  to  the  parent  bed  or  its  neighborhood. 
Ores  introduced  by  circulating  waters  into  strata,  subse- 
quent to  their  formation,  often  choose  one  bed  in  preference 
to  another  in  consequence  of  some  chemical  or  physical 
peculiarity  favorable  to  deposition:  and  thus  the  resultant 
ore-body  takes  on  a  bedded  form. 

Example:  In  Piemonte,  near  Brosso,  Italy,  are  beds  of 
specular  iron  (hematite)  and  pyrite  regularly  interstratified 
with  beds  of  limestone  and  mica-schist.  Nearby,  at 
Traversella,  are  deposits  of  pyrite,  magnetite  and  copper 
pyrite  in  dolomite.  At  both  these  places  the  deposits  are 
confined  to  the  flanks  of  an  intrusive  quartz-bearing  diorite, 
and  the  chief  ore-deposits  are  accompanied  by  garnet  or 
hornstone  (altered  and  hardened  limestone),  together  with 
other  metamorphic  rocks.  The  conclusion  has  hence  been 
reached,  that  the  ores  are  due  to  the  influence  of  the 
intrusive  rock;  that  first  gases,  and  later  hot  springs,  both 
emanating  from  the  diorite,  attacked  and  mineralized  the 
intruded  strata.  The  limestone  strata  were  more  strongly 
affected  than  the  mica-schists  and  were  replaced  by  ores, 
so  that  they  now  are  represented  by  ore-beds  intercalated 
in  the  schists.  Strong  fractures  permitted  the  gases  and 
water  to  penetrate  far  from  the  diorite,  so  that  the  mineral- 
ization was  extensive.* 

7s  it  important  to  tell  whether  a  bed  of  ore  is  primary  or 

secondary? 

Such  a  distinction  is  important,  for  the  reason  that  in  the 
first  case  the  ore  will  invariably  follow  its  regular  bed, 

*  V.  Novarese,  Bull.  Com.  Geol.  Itai.  Vol.  XXXII,  pp.  75-93,  1901. 


60  GEOLOGY  APPLIED  TO  MINING. 

while  in  the  second  we  must  be  always  expecting  it  to 
deviate  from  it  or  to  occur  in  other  forms.  This  latter 
caution  must  also  be  maintained  in  regard  to  those  primary 
bedded  deposits  which  have  undergone  secondary  concen- 
tration by  circulating  waters.  These  waters,  besides  con- 
centrating the  ore  within  the  parent  bed,  are  likely  to 
carry  it  out  and  to  form  ore-deposits  at  a  distance  from  it. 

What  chief  points  must  one  keep  in  mind  in  following  an 

ore-bearing  stratum? 

In  any  case  one  of  the  chief  things  is  to  be  able  to  trace 
and  recognize  the  same  bed  in  different  places.  Whether 
the  occurrence  of  ore  in  bedded  form  is  primary  or  sec- 
ondary, it  must  be  associated  with  a  fairly  constant 
character  of  the  rock.  If  the  deposit  is  original,  the  con- 
ditions which  brought  about  the  deposition  of  the  same 
valuable  mineral  in  various  places  must  have  given  rise  to 
other  uniform  physical  characters.  If  it  is  secondary^ 
the  physical  or  chemical  character,  which  determined 
the  precipitation  of  ore  along  a  certain  bed,  must  be 
present  wherever  we  can  reasonably  look  for  a  continuance 
of  that  ore  If  a  limestone  bed  containing  replacement 
deposits  passes  laterally  into  a  sandstone,  the  ore  may  be 
poor  or  wanting  in  the  sandstone.  If  a  shale  bed,  which  by 
its  impermeability  or  its  organic  matter  has  determined  the 
deposition  of  ores  in  or  near  it,  passes  into  a  sandstone  or 
limestone,  again  we  must  look  for  a  change,  and,  very 
likely,  the  disappearance  of  this  ore-horizon.  For  this 
reason,  in  tracing  an  ore-horizon,  physical  and  chemical 
points  are  among  the  most  valuable  means  of  identification, 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  61 

and  where  such  points  fail  the  chances  are  that  the  ore- 
horizon  fails  too. 

Do  ore-bearing  strata  ever  extend  as  such  for  long  distances? 

Often  a  certain  bed  may  be  traced  by  its  physical  char- 
acters hundreds  of  miles,  and  is  everywhere  a  valuable 
indication  of  contained  ore. 

Example:  In  the  Lake  Superior  district,  the  ore-deposits 
are  confined  to  a  certain  set  of  beds,  easily  recognizable 
by  their  peculiar  physical  characters.  These  beds  make 
up  the  iron-bearing  formation — they  are  sometimes  slaty, 
sometimes  massive,  generally  dark-colored  rock — and  by 
tracing  them  vast  ore-deposits  are  continually  found. 
This  point  is  brought  out  in  the  following  quotation  from 
Mr.  Oscar  Rohn.* 

"It  may  be  well  to  recall  that  the  iron  ore-deposits  of  the 
Lake  Superior  district  always  occur  in  certain  character- 
istic formations,  called  iron-bearing  formations,  which  are 
associated  with  a  series  of  conglomerates,  quartzites, 

slates,  and  vein  stones So  well  understood 

and  so  generally  recognized  is  the  association  of  ore-deposits 
with  a  characteristic  rock  formation  that  in  all  well- 
directed  prospecting  the  limits  of  the  formation  are  sought, 
and  underground  work  confined  to  the  area  within  these 
limits." 

How  may  the  presence  of  a  known  ore-bearing  stratum  be 
recognized  in  a  place  where  it  does  not  outcrop? 
The  relation  of  an  ore-bearing  bed  to  other  beds  lying 

*  Engineering  and  Mining  Journal,  Vol.  LXXVI,  p.  616. 


62  GEOLOGY  APPLIED  TO  MINING. 

above  and  below  should  be  studied  and  borne  in  mind. 
Often  where  the  ore-bearing  bed  is  not  exposed  at  the 
surface,  or  is  covered  with  surface  debris,  the  recognition 
of  a  bed  having  a  known  relation  to  the  ore-bed  leads  to 
the  discovery  of  the  latter  and  its  contained  ore-deposits. 

PREFERENTIAL    ASSOCIATION    WITH    CERTAIN    GEOLOGIC 
PERIODS. 

Are  certain  minerals  preferentially  associated  with  certain 

geologic  periods? 

To  a  limited  and  unreliable  extent,  even  in  the  most 
favorable  cases:  generally  not  at  all.  Formerly  this 
association  was  thought  to  be  very  important,  however. 

A  famous  and  able  geologist,  Sir  Roderick  Murchison,  as 
late  as  1851  and  1859,  made  the  statement  that  the  chief 
sources  of  gold  were  in  Paleozoic  rocks,  particularly  in  the 
Lower  Silurian, — an  opinion  largely  based  on  a  fragmentary 
knowledge  of  Australian  geology.  Since  then,  in  Australia, 
gold  has  been  worked  in  Carboniferous,  Devonian  and 
Silurian  rocks,  and  even  in  the  Triassic  and  Jurassic;  in 
South  Africa  the  gold  is  largely  in  Devonian  strata. 

However,  when  we  come  to  consider  the  world's  great 
mining  regions,  we  observe  that  it  is  indeed  the  older 
strata  which  in  many  cases  carry  the  ores.  The  older 
rocks  have  had  more  time  than  the  younger  ones  and 
more  opportunity  to  become  the  seat  of  ore-deposition; 
they  have  also  been,  in  most  cases,  deeply  buried  by  the 
younger  strata  and  so  brought  under  such  physical  con- 
ditions of  heat  and  pressure  as  are  conducive  to  ore-con- 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  63 

centration;  under  these  conditions  they  have  been 
pierced  by  intrusive  rocks  and  subjected  to  all  the  ac- 
companying processes  of  alteration  and  concentration. 

To   what  geologic  period  do  metalliferous  veins  generally 

belong? 

Metalliferous  veins  seemed  to  have  formed  at  every 
period  of  the  world's  history.  As  shown  in  the  first 
chapter,  they  are  the  result  of  concentrating  by  agencies 
which  have  been  active  since  the  earliest  age  down  to 
the  present  day.  The  age  of  veins,  as  actually  proven, 
is  extremely  various,  ranging  from  the  Archaean  through 
the  Paleozoic,  Mesozoic  and  Tertiary.  Passing  over  the 
cases  of  veins  and  other  ore-bodies  belonging  to  older 
periods,  we  may  mention  that,  in  America,  the  rich  silver 
districts  of  Leadville  and  Aspen,  Colorado,  and  Eureka, 
Nevada,  are  of  Tertiaiy  age.  Moreover,  in  the  Monte  Cristo 
district,  Washington,  the  present  writer  has  reached  the 
conclusion  that  the  veins  are  chiefly  Quaternary  (Pleisto- 
cene). At  Steamboat  Springs,  in  Nevada,  the  actually 
escaping  waters  have  deposited  silica  along  the  fissures 
which  they  traverse.  This  silica  often  contains  sulphide 
of  mercury,  in  some  quantity,  and  traces  of  gold.  Similar 
phenomena  have  been  noted  in  other  places. 

Example:  Workable  deposits  of  cinnabar  and  stibnite 
(sulphides  of  mercury  and  antimony)  at  Monte  Amianta 
and  other  places  in  Tuscany,  Italy,  have  formed  subse- 
quent to  early  Pleistocene  volcanic  eruptions  (andesite 
and  trachyte).  The  cinnabar  is  found  in  the  volcanic  rock 
as  well  as  the  associated  sediments,  and  has  formed  by 


64  GEOLOGY  APPLIED  TO  MINING. 

preference  in  limestone  beds.  The  stibnite  is  closely 
connected  with  the  cinnabar,  and  is  associated  with  arsenic 
sulphide  (realgar),  pyrite  and  limonite,  and  sulphur. 
Most  of  the  ore-deposits  are  intimately  connected  with 
sulphur  springs  or  emanations  of  sulphuretted  hydrogen, 
which  are  accompanied  by  incrustations  of  sulphur,  and 
silicious  sinter  which  sometimes  contains  cinnabar.* 

In  opened  mines  and  in  placers  it  has  been  found  that 
surface  waters  still  precipitate  all  sorts  of  metals,  even  gold, 
from  solution. 

Example:  The  formation  of  lead  and  zinc  sulphides  is 
now  taking  place  in  the  Missouri  mines.  An  instance  may 
be  cited  where  an  old  tunnel  near  Joplin,  driven  through 
shales,  became  filled  with  water  and  was  left  so  for  ten  or 
twelve  years.  In  1898,  when  it  was  re-opened,  the  surface 
of  the  shales,  on  the  roof  and  sides  of  the  tunnels,  was 
found  to  be  thickly  encrusted  with  minute  crystals  of 
blende.  In  places,  the  blende  was  deposited  on  the  pick- 
marks  made  when  the  tunnel  was  run.  j 

Can  any  statement  be  made  concerning  the  most  favorable 

geological  age  for  ore-deposition? 

The  ore-deposits  of  earlier  ages  have  been  exposed,  along 
with  the  containing  rocks,  to  destruction  by  erosion;  and, 
even  where  the  containing  rocks  have  not  been  removed, 
circulating  waters  have  very  often  attacked  the  ores  and 
transformed  them  wholly  or  partly  into  secondary  or  sub- 
sequent deposits.  On  the  other  hand,  ores  of  compara- 

*B.  Lotti,  Zeitschrift  }ur  praktische  Geologic,  1901,  pp.  41-46. 
tW.  P.  Jenney,  Transactions  American  Institute  Mining  Engineers,  Oct., 
1902,  p.  26. 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  65 

lively  recent  date,  such  as  those  of  the  Tertiary,  will,  in 
mountain  regions,  have  undergone  only  the  right  degree  of 
erosion  for  laying  them  open  to  the  eye  of  man.  So  it  is 
possible  that  the  younger  ore-deposits  will  be  found  to 
preponderate  over  the  older  ones,  among  the  important 
districts  actually  exploited. 

That  is  to  say, — other  things  being  equal,  important  ore- 
deposits  are  rather  more  likely  to  occur  in  rocks  belonging  to 
the  older  geologic  ages  than  in  the  younger  ones;  and  their  date 
of  formation  is  more  apt  to  belong  to  the  younger  geologic  ages 
than  to  the  older  ones. 

What  is  in  general  the  age  of  primary  bedded  deposits? 

In  river  beds  and  on  sea-beaches,  gold-placers,  tin- 
placers,  etc.,  are  today  being  formed,  as  they  have  been  in 
past  ages.  Most  of  the  world's  productive  placers  are 
Quaternary,  for  these  are  easily  found  and  have  not  been 
destroyed  by  erosion,  as  is  the  case  with  many  of  the  older 
surface  deposits.  Yet  in  California,  British  Columbia,  and 
Australia,  Tertiary  gravels,  especially  when  protected  by 
overflows  of  lava,  have  afforded  immense  wealth.  Similar 
deposits  exist  in  various  older  formations.  Some  author- 
ities consider  the  Witwatersrand  gold-bearing  conglomer- 
ates, which  occur  amid  a  Devonian  formation,  as  beach 
placers  of  that  period;  and  placers  of  Cambrian  and  even 
pre-Cambrian  age  have  been  described. 

At  the  present  day,  also,  iron,  manganese,  gypsum,  etc., 
are  being  chemically  precipitated  in  bed  form  in  the  bottoms 
of  lakes,  seas  and  oceans ;  and  this  process  is  known  to  have 
been  active  during  the  past  ages. 


66  GEOLOGY  APPLIED  TO  MINING. 

7s  it  true  that  different  geologic  ages  have  no  special  charac- 
teristics as  regards  mineral  deposits? 

It  seems  to  be  the  case  that  in  the  different  geologic  ages 
the  general  conditions  varied  more  or  less  uniformly;  and 
in  some  of  these  ages  the  conditions  for  forming  certain 
mineral  deposits  were  better  than  in  others. 

Are  coal  and  oil  deposits  confined  to  certain  geologic  periods? 

At  certain  periods  of  the  earth's  history  vegetation 
flourished  very  rankly  and  swamps  were  very  abundant, 
enabling  the  preservation  of  accumulated  layers  of  vegetable 
matter,  which  thus  became  a  part  of  the  strata  of  that 
period,  and,  by  consolidation  and  metamorphism,  have 
turned  into  lignite,  soft  or  hard  coal.  The  Carboniferous 
period  was  favorable  to  this  process,  and  much  coal  was 
formed  then.  It  was  thought,  indeed,  at  one  time,  that 
coal  was  formed  only  in  this  period,  but  since  then  it  has 
been  found  in  quantity  in  other  formations.  In  Virginia, 
there  is  good  coal  in  Triassic  strata.  In  the  Western 
United  States,  great  quantities  of  coal  are  found  in  the 
Cretaceous,  and  on  the  Western  coast,  especially  in  Alaska, 
it  is  abundant  in  the  Tertiary.  In  Alaska  we  can  see  at 
the  present  day  the  first  process  of  coal  formation  in  the 
great  areas  of  swamp-peat,  which  is  often  many  feet  thick, 
and  shows  the  closest  resemblance  in  habit  to  the  late 
Tertiary  lignites  of  the  same  region.  When  these  peat- 
swamps  shall  have  been  covered  up  by  later  strata,  and 
consolidated  and  changed  by  pressure,  they  will  become 
coal-beds. 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  67 

Thus  we  see  that  we  cannot  confine  the  formation  of 
coal-beds  to  any  one  geologic  period.  Yet  we  may  still 
regard  the  Carboniferous  rocks  as  especially  likely  to 
contain  such  deposits,  while  we  should  hardly  expect  to 
find  good  coal-seams  in  the  Cambrian  and  Silurian. 

These  favorable  conditions,  during  a  certain  period,  were 
not  world  wide.  At  one  place  was  marshy  land,  at  another 
the  ocean.  In  the  Carboniferous,  over  eastern  North 
America,  there  was  much  land,  lagoons  and  flourishing 
vegetation;  in  the  western  United  States  there  was  gener- 
ally deep  sea.  So  in  the  eastern  area  we  -find  coal  plenti- 
fully in  the  Carboniferous,  especially  in  certain  beds, 
which  can  sometimes  be  traced  for  hundreds  of  miles;  while 
in  the  western  area  the  Carboniferous  was  not  especially 
a  coal-bearing  period.  Conversely,  in  the  Rocky  Mountain 
region  the  Cretaceous  is  the  great  coal-bearing  horizon;  but 
this  is  not  the  case  in  the  eastern  United  States. 

Oil  is  another  mineral  for  which  the  conditions  of  forma- 
tion have  been  more  favorable  in  certain  periods  than  in 
others.  Abundance  of  organic  life  during  these  periods 
seems  to  have  been  the  favoring  circumstance,  for  the 
organic  matter  has  by  its  decomposition  formed  the  oil. 

Are  there  minerals  other  than  coal  and  oil  which  were  depos- 
ited more  abundantly  in  certain  periods? 

In  some  periods,  more  than  in  others,  there  appear  to 
have  been  large  areas  of  shallow  evaporating  seas,  from 
which  certain  mineral  substances  were  precipitated.  Thus, 
both  in  the  old  world  and  in  the  new,  Permian  strata  (the 


68  GEOLOGY  APPLIED  TO  MINING. 

youngest  part  of  the  Carboniferous  age)  contain,  in  many 
places,  deposits  of  salt,  gypsum,  etc. 

Is  a  knowledge  of  fossils  of  value  in  identifying  the  same  ore- 
bearing  stratum  in  different  districts? 
Besides  the  identification  of  an  ore-bed  by  its  physical 
characteristics,  it  is  frequently  possible  to  recognize  it  by 
means  of  its  contained  fossils.  The  fossils  are  the  only  safe 
evidence  of  identity  in  age,  when  the  districts  to  be  com- 
pared lie  some*  distance  apart.  Beds  having  the  same 
physical  appearance  may,  and  dol  occur  in  many  ages, 
while  perhaps  only  in  one  did  there  exist  the  finer  condi- 
tions which  were  favorable  to  the  production  of  deposits  of 
valuable  minerals. 

Example:  Dr.  Le  Neve  Foster  relates  that  a  French 
inspector  of  mines.  M.  Meugy,  hearing  of  the  discovery  of 
phosphate  of  lime  in  a  certain  part  of  the  Cretaceous  in 
England,  and  knowing  from  fossil  evidence  that  beds  of  the 
same  age  existed  in  France,  concluded  that  the  French 
beds  might  also  contain  phosphate  deposits — a  conclusion 
which  was  amply  verified  by  prospecting. 

PREFERENTIAL   ASSOCIATION    WITH    CERTAIN    KINDS    OP 
SEDIMENTARY  ROCKS. 

Are  certain  kinds  of  minerals  preferentially  associated  with 

certain  kinds  of  sedimentary  rocks? 

Certain  kinds  of  rocks  are  sometimes  preferred  by  certain 
minerals  for  deposition;  but  there  is  no  regular  rule.  The 
association  may  be  either  primary  or  secondary. 


ARRANGEMENT  OF  STRATIFIED  ROCKS.  69 

CONTEMPORANEOUS  DEPOSITION  OF  ORES  AND  STRATA. 

In  what  cases  is  such  association  primary  f 

In  coarse  sediments  which  are  evidently  shallow  water 
or  shore-formations,  such  as  coarse  impure  sandstones, 
conglomerates  and  clays,  mixed  with  vegetable  material 
and  plant  remains,  we  may  suspect  coal  or  oil,  or  natural 
gas. 

Example:  The  petroleum-bearing  strata  of  all  periods 
and  of  all  parts  of  the  world  show,  according  to  Dr.  Rudolf 
Zuber,*  a  remarkable  resemblance  in  their  formation  and 
composition.  Everywhere  they  are  bituminous  clay- 
shales,  and  variegated,  mostly  bright-colored,  clays, 
interstratified  with  sandstones  and  conglomerates.  Lime- 
stones, which  may  also  occur  in  such  series,  contain  tarry 
materials,  but  rarely  true  petroleum. 

There  is  a  surprising  resemblance  between  the  red  and 
green  clays  and  shales  of  an  American  Paleozoic  oil-bearing 
formation,  with  oil-bearing  formations  in  the  Jurassic  of 
Germany,  and  in  the  Eocene  of  Galicia.  The  oil-bearing 
strata  of  the  Upper  Triassic  in  the  western  part  of  the 
Argentine  Republic  are  like  the  middle  Tertiary  (Oligocene) 
oil-bearing  rocks  of  the  Carpathian  Mountains;  the  green 
oil-bearing  shales,  lying  between  the  Jurassic  and  the 
Cretaceous,  in  the  northern  part  of  the  Argentine  Republic, 
cannot  be  distinguished  from  the  Galician  Eocene  strata. 
The  oil-bearing  strata  at  Baku  in  the  Caucasus  are  identical 
in  appearance  with  the  Carpathian  Oligocene  shales  and 
sandstones. 

*  Zeitschrift  fur  praktische  Geologic,  March,  1898,  p.  85. 


70  GEOLOGY  APPLIED  TO  MINING. 

Deposits  of  salt,  gypsum,  etc.,  may  be  looked  for  espe- 
cially in  connection  with  red  sandstones.  This  association 
has  been  explained  by  the  fact  that  when  sea-water  evapo- 
rates, the  first  precipitate  is  oxide  of  iron  (which  gives  the 
red  color  to  the  rocks) ;  this  is  followed  by  gypsum  and  then 
by  salt. 

Can  one  admit  such  an  association  for  metallic  minerals? 

It  is  as  yet  an  open  question  as  to  whether  copper  may 
not  also  be  admitted  to  the  same  association.  In  Germany 
the  Mansfeld  Kupferschiefer  (copper-slate)  is  a  thin  bed  of 
bituminous  shale  lying  between  two  thick  deposits  of 
Permian  sandstone.  The  ore  contains,  besides  copper, 
silver,  lead,  zinc,  antimony,  mercury,  nickel  and 
cobalt.  This  Kupferschiefer  outcrops  over  a  large  district 
and  frequently  there  is  copper  either  in  it  or  in  the  other 
beds  of  the  Permian.  At  St.  Avoid  and  Wallerfangen, 
(also  in  Germany),  copper  ores  which  occur  in  Triassic 
sandstone,  have  been  supposed  to  be  contemporaneous 
with  the  enclosing  rock.  In  Utah,  in  the  Silver  Reef 
district,  red  and  gray  Triassic  sandstones  and  shales  contain 
bedded  copper-silver  deposits.  In  the  Nacimiento  mount- 
ains in  New  Mexico  copper  ores  occur  in  Triassic  sandstones, 
associated  with  plant  remains.  Similar  deposits,  although 
without  the  organic  remains,  have  been  described  by  Prof. 
W.  P.  Blake  as  occurring  in  Arizona.  Prof.  James  F. 
Kemp  gives  many  examples  of  copper  ores  in  Triassic  and 
Permian  sandstones.  "Copper  ores,"  he  says,  "are  very 
common  throughout  the  estuary  Triassic  rocks  of  the 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  71 

Atlantic  coast."*  In  the  Permian  of  northern  central 
Texas  there  are  three  separate  copper-bearing  zones, 
forming  three  lines  of  outcrop  that  extend  in  a  general 
northeasterly  direction  over  a  range  of  about  three  counties. 

May  not  these  bedded  deposits  of  metallic  minerals  have  been 
introduced  into  the  beds  subsequent  to  their  deposition? 
Some  observers  have  reasoned  that  the  above  deposits 
were  deposited  contemporaneously  with  the  other  strata; 
while  others,  finding  evidence  of  the  concentration  of  the 
ores  especially  along  fault-planes  and  other  water-channels, 
have  believed  that  the  metals  were  introduced  from 
extraneous  sources  through  these  channels,  and  are  only  in 
bed  form  because  the  layers  of  organic  material  served  to 
precipitate  the  metals  in  the  ascending  waters.  The  present 
writer  believes  that  the  theory  of  original  precipitation 
contemporaneously  with  the.  strata  has  strong  features  to 
recommend  it,  even  though  he  regards  a  later  concentration 
by  circulating  waters  as  proven  in  many  cases. 

//  these  bedded  deposits  are  even  in  part  original,  where  did 
the  metals  come  from,  and  how  were  they  precipitated? 

Organic  matter  is  known  to  be  a  powerful  precipitant  of 
metals  from  solution.  Metals  are  known  to  exist  in  sea- 
water,  even  gold.  According  to  Phillips, f  the  waters  of 
the  Mediterranean  contain  one  centigram  of  copper  to  the 
cubic  meter.  The  same  writer  remarks  that  the  "black 
and  usually  very  sulphurous  matter  deposited  in  basins 

*  'Ore  Deposits  of  the  United  States  and  Canada.'     Fourth  Edition,  p.  223. 
f  'Ore  Deposits,'  Second  Edition,  p.  132. 


72  GEOLOGY   APPLIED   TO    MINING. 

where  sea-water  has  been  left  to  itself  constantly  contains 
copper,  and  the  same  is  generally  true  with  regard  to  the 
dark-colored  gypseous  muds  of  all  ages."  Luther  Wagoner* 
found  that  mud  from  San  Francisco  Bay  contained  gold 
and  silver;  and  that  samples  rich  in  organic  matter  con- 
tained more  than  at  other  places.  He  concluded  that 
organic  matter  in  mud  reduces  some  silver  from  the  sea- 
water,  and  probably  some  gold. 

Example:  In  the  Sierra  Oscura,  New  Mexico,  are  red 
sandstones  and  shales  of  probably  Permian  age.  Some  of 
these  red  sandstone  beds  contain  copper  for  an  extent  of  a 
number  of  miles.  In  a  certain  portion  of  the  region  there 
are  at  least  three  distinct  copper-bearing  sandstones,  in 
which  the  ore  is  chalcocite  and  copper  carbonate,  dissem- 
inated in  minute  grains.  No  dikes  or  igneous  rocks  of  any 
kind  have  been  found  associated  with  the  copper-reefs  or 
the  enclosing  beds.  They  do  not  occupy  lines  of  faulting; 
and,  indeed,  were  certainly  formed  before  the  main  faults 
of  the  district.  The  mode  of  occurrence  of  the  ore  in 
regular  beds,  in  part  replacing  plant  remains,  suggests  that 
the  copper  was  deposited  from  the  waters  which  deposited 
the  enclosing  sediments,  f 

How  can  one  explain  the  hypothesis  that  certain  geologic 
periods  were  more  favorable  for  this  process  than  others? 
It  may  well  be  that  during  periods  like  the  Permian  and 
Triassic,  where  the  presence  of  great  land-locked  evapo- 
rating shallow  seas  is  shown  by  beds  of  gypsum,  salt,  etc., 
with  impure  red  sandstones,  the  precipitation  of  metals  in 

*  Transactions  American  Institute  Mining  Engineers,  Vol.  XXXI   p.  807. 
fH.  W.  Turner,  Transactions  American  Institute  Mining  Engineers,  Oct., 
1902. 


ARRANGEMENT    OP    STRATIFIED    ROCKS.  73 

the  muds  was  greater  than  at  other  times.  In  the  first 
place,  the  evaporation  of  the  sea-water  would  concentrate 
the  metals,  along  with  the  other  substances  in  solution;  in 
the  second  place,  the  shore-line  conditions  would  furnish 
beds  rich  in  organic  matter,  for  the  reduction  and  precipita- 
tion of  these  metals. 

SELECTION  OF  FAVORABLE  STRATA  FOR  THE  SUBSEQUENT 
DEPOSITION  OF  ORES. 

Subsequent  bedded  deposits,  where  the  minerals  were 
introduced  after  the  formation  of  the  rocks,  show  frequently 
a  preference  for  one  kind  of  rock. 

Do  sandstones  especially  attract  the  precipitation  of  ores? 

Porous  sandstones  afford  channels  for  waters,  and  many 
tiny  cavities  which  may  be  filled  with  precipitated  minerals; 
hence  they  are  often  selected  for  ore-deposition  in  prefer- 
ence to  adjoining  strata. 

Do  schists  especially  attract  the  deposition  of  ores? 

Schists  are  often  selected  for  impregnation  with  valuable 
minerals  for  the  same  reasons  as  sandstones;  moreover, 
shales,  slates  and  schists  containing  organic  matter  often 
act  as  precipitants  to  ore-bearing  solutions,  and  hence 
become  the  seat  of  ore-deposition  in  preference  to  other 
beds. 

Example:  A  small  basin  of  coal-shales,  near  Belleville, 
Jasper  county,  Missouri,  carries  beautifully  preserved  fossil 
plants.  The  outer  surface  of  the  mass  of  shales,  for  a 


74  GEOLOGY  APPLIED  TO,  MINING. 

depth  of  about  a  foot,  contains  scattered  crystals  of  blende, 
with  some  galena  and  pyrite.  The  central  portion  does  not 
carry  any  mineral,  the  mass  having  been  mineralized  from 
the  outside,  toward  the  interior,  so  far  only  as  the  mineral- 
bearing  waters  could  penetrate  the  dense  plastic  clay.  The 
crystals  in  their  growth,  have  distorted  otherwise  perfect 
fossil  plants,  giving  evidence  that  the  deposition  of  the 
minerals  was  of  later  date  than  the  embedding  of  the  plant 
remains.* 

How  does  a  stratum  impervious  to  water  cause  the  precipita- 
tion of  ores  in  bedded  form? 

A  porous  stratum  may  absorb  the  waters  coming  to  it 
from  transverse  fissures  or  other  channels,  may  spread 
these  waters  out,  make  their  circulation  sluggish,  and  so 
induce  the  precipitation  of  ores.  An  impermeable  stratum, 
such  as  a  bed  of  shale,  may  refuse  all  passage  to  solutions, 
and  compel  them  to  slacken  and  spread  out  on  its  upper 
or  under  side  (according  to  whether  the  waters  are  ascend- 
ing or  descending),  and  likewise  bring  about  precipitation. 

Example:  In  a  series  of  interbedded  '  sandstones  and 
shales  at  Rico,  Colorado,  occurs  a  bed  relatively  impervious 
to  water— the  so-called  "blanket"  (Fig.  4).  In  the  case  of 
the  Enterprise  mine,  the  blanket  seems  to  have  been 
formed  from  the  fine  residue  left  from  the  dissolution  of  a 
gypsum  stratum,  together  with  a  breccia  made  by  the 
collapse  of  the  overlying  beds,  as  the  gypsum  disappeared. 
The  rocks  have  been  seamed  by  nearly  vertical  fissures, 
and  mineralizing  solutions  rising  along  these  have  been 


*  W.  P.  Jenney,  Transaction*  American  Institute  Mining  Engineers,  Oct., 
1902,  p.  25. 


ARRANGEMENT    OP    STRATIFIED    ROCKS.  75 

checked  by  the  blanket,  and  have  deposited  their  ores  in  it, 
on  its  under  side.  The  ore-deposits,  therefore,  have  a 
constant  relation  to  this  particular  bed,  but  the  association 
is  a  subsequent  one,  the  ores  having  been  introduced  after 
the  deposition  of  the  sediments. 


Fig.  4.  Longitudinal  section  through  the  Group  tunnel,  Enterprise  mine,  Rico, 
Colo.     After  F.  L.  Ransome. 

Why  may  ores  be  deposited  in  a  rigid  stratum  in  preference  to 

soft  strata? 

A  certain  stratum  may,  on  account  of  its  rigidity,  be 
favorable  to  the  formation  of  open  fractures,  which  cease  in 
neighboring  soft  beds.  Hence  veins  formed  along  such 
fractures  may  be  confined  chiefly  to  the  more  rigid  strata. 

Why  is  it   that   subsequent   ore-deposits  frequently  show  a 
preference  for  limestones  over  associated  rocks? 
Limestone  lends  itself  readily  to  the  process  of  cavity- 
making  by  the  dissolving  action  of  waters,  more  than  any 
other  rock,  and  these  cavities  are  sometimes  filled  up  by 
minerals.     Yet  true  cave  deposits  are  probably  much  rarer 
than  they  were  formerly  thought  to  be.     At  Eureka,  in 


76  GEOLOGY  APPLIED  TO  MINING. 

Nevada,  rich  ores  occur  in  caves,  but  it  seems  that  the 
caves  have  been  formed  by  the  shrinkage  of  ore-bodies, 
attendant  upon  their  alteration. 

Limestone  is  easily  replaced  by  metallic  minerals,  and 
for  this  reason  is  frequently  chosen  above  other  rocks,  by 
circulating  waters,  as  a  locus  for  the  precipitation  of  the 
metals  they  carry.  Silver-lead  deposits,  especially,  prefer 
limestone.  Iron  deposits  also  frequently  choose  this  rock. 

Example:  The  superior  suitability  of  limestones  over 
other  rocks  for  replacement  deposits,  particularly  of  lead, 
is  shown  in  the  case  of  the  Derbyshire  lead  mines  in 
England.  In  this  district  there  occur,  in  limestone, 
intrusive  sheets  of  igneous  rock.  Fractures  traverse  both 
limestone  and  intrusive  rock,  and  these  fractures,  together 
with  the  joints  and  the  bedding  planes  of  the  limestones, 
have  evidently  furnished  the  channels  along  which  the 
ore-bearing  solutions  have  circulated.  The  galena  which 
constitutes  the  ore,  however,  has  been  deposited  only  in 
the  limestone,  and  in  the  intrusive  rock  the  fractures  are 
barren.  (Fig.  5.) 


Fig.  5.  Lead  deposits,  Derbyshire,  England.     After  De  La  Beche.      a.=lime- 
stone;  b.=igneous  rock ;  c.=ore. 

How  may  the  chemical  peculiarities  of  certain  strata  induce 
the  precipitation  of  ores  in  them? 
Besides  the  chemical  suitability  of  limestone  for  replace- 


ARRANGEMENT    OF    STRATIFIED    ROCKS.  77 

ment,  and  the  precipitating  action  of  beds  containing 
organic  matter,  other  peculiarities  of  certain  stratified 
rocks  may  induce  ore-deposition. 

In  shale  beds  there  is  always  a  considerable  percentage  of 
iron.  This  usually  combines  with  sulphur  contained  in 
organic  matter  to  form  sulphide  of  iron  (pyrite).  Pyrite 
has  been  shown  by  experiment  to  be  active  in  causing  other 
metallic  minerals,  especially  gold,  to  precipitate. 

Example:  At  Ballarat,  in  Australia,  there  are  certain 
thin  beds  of  slate  containing  pyrite.  These  are  called 
"indicators,"*  for,  whenever  the  auriferous  quartz  veins 
of  the  district  intersect  them,  the  veins  become  uniformly 
rich,  even  though  in  the  remainder  of  their  course  they  will 
not  repay  working.  The  recognition  and  tracing  out  of 
these  indicator  beds,  therefore,  becomes,  perhaps,  the 
most  important  geologic  work  for  mining  men  in  these 
districts.  Even  when  the  beds  have  been  bent  and  broken 
by  earth  movements  their  effect  upon  traversing  lodes  is 
the  same. 

In  sum,  then,  what  sedimentary  rocks  are  most  likely  to  become 

the  site  of  ore-deposition  from  circulating  waters? 

Among  the  stratified  rocks,  now  one,  now  another,  may 

by  its  peculiarities  induce  its  selection  in  preference  to 

adjoining  strata,  as  a  site  of  ore-deposition.     The  problem 

admits  of  no  empirical  solution,  but  in  each  individual  case 

a  comparison  of  the  physical  and  chemical  features  of  each 

bed  present  may  aid  in  deciding  which  is  most  favorable, 


*  T.  A.  Rickard.     'The  Indicator  Vein,   Ballarat,  Australia.'      Transactions 
American  Institute  Mining  Engineers,  Vol.  XXX,  pp.  1004-1019. 


78  GEOLOGY   APPLIED   TO    MINING. 

and  in  discovering  the  main  mineral  belt.  Shale  beds  con- 
taining organic  matter  (especially  if  the  beds  be  limy  and 
so  easily  adapted  to  the  process  of  replacement)  offer  per- 
haps the  most  favorable  situation  for  ores.  Next  in 
order  comes  limestones,  while  conglomerates  are  in  general 
not  so  favorable.  Quartzites  are  the  least  favorable  of 
ail,  although  even  here  ore-bodies  are  not  infrequent. 


CHAPTER  III. 

THE  STUDY  OF  IGNEOUS  ROCKS  AS  APPLIED  TO 
MINING. 


PHYSICAL  CHARACTERS  OF  IGNEOUS  ROCKS. 

What  is  an  igneous  rock? 

A  rock  is  an  aggregate  of  minerals. 

An  igneous  rock  is  one  that  has  cooled  from  a  molten 
condition. 

How  can  one  tell  if  a  rock  is  igneous  or  not? 

An  igneous  rock  bears  marks  indicating  its  origin,  just 
as  the  stratified  and  sedimentary  rocks  do.  In  the  first 
place,  since  igneous  rocks  were  not  deposited  in  successive 
layers  in  water,  there  is  no  true  stratification. 

Do  igneous  rocks  never  have  a  banded  structure? 

In  the  case  of  lavas,  when  one  flow  after  the  other  is 
poured  out,  the  accumulated  rocks  will  be  definitely  lay- 
ered, for  between  each  flow  there  may  be  fragmental  broken 
material.  Each  flow  also  may,  perhaps,  differ  slightly  in 
texture  and  composition  from  the  earlier  and  the  later 
flows,  and  within  itself  the  top  and  bottom  will  generally 
be  finer  grained  than  the  center,  on  account  of  having 


80  GEOLOGY  APPLIED  TO  MINING. 

cooled  more  quickly.  When  an  igneous  rock  flows  in  a 
viscous,  only  incompletely  molten  condition,  as  it  frequently 
does,  it  may  be  drawn  out  into  long  bands  differing  more  or 
less  from  one  another  in  structure,  texture  and  compo- 
sition. These  bands  may  be  a  fraction  of  an  inch  or  many 
feet  in  thickness.  This  structure  is  flow-banding. 

Does  the  crystalline  structure  afford  a  test  for  igneous  rocks? 

The  structure  of  the  igneous  rock, when  closely  examined, 
is  the  best  test  of  its  origin.  In  the  first  place,  it  is  usually 
crystalline.  On  hardening  from  a  molten  or  otherwise 
fluid  condition  solid  substances  tend  to  assume  the  beautiful 
and  often  highly  complicated  geometric  forms  which  are 
peculiar  to  them.  In  the  molten  state  all  the  elements  are 
intimately  mingled — as  consolidation  commences  they 
begin  to  group  themselves  together  according  to  their 
affinities,  and  to  form  certain  combinations  which  we  call 
minerals,  each  having  a  crystal  form  mathematically 
distinct  from  that  of  unlike  minerals. 

Are  all  igneous  rocks  thus  made  up  of  crystalline  minerals? 

If  the  consolidation  is  extremely  sudden,  this  crystal- 
lization has  not  time  to  take  place  (at  least  we  cannot 
detect  it,  even  with  a  microscope),  or  is  shown  only  by 
beautiful  groupings  of  the  rock  materials,  with  no  definite 
separation  into  minerals.  The  rock,  thus  quickly  hardened, 
is  termed  volcanic  glass  or  obsidian. 

Example:  Obsidian  cliff,  in  the  Yellowstone  National 
Park,  rises  in  nearly  vertical  walls  for  150  or  200  feet.  It 


IGNEOUS     ROCKS.  81 

has  been  formed  as  a  surface  flow,  and  is  a  natural  glass, 
the  result  of  rapid  cooling  of  a  fused  mass  of  rhyolitic  lava. 
This  glass  covers  an  area  of  about  10  square  miles.* 

Is  this  glassy  condition  of  igneous  rocks  usual? 

Nearly  always  the  cooling  is  slow  enough,  even  in  the 
case  of  lavas  suddenly  poured  out,  to  allow  at  least  the 
beginning  of  crystallization.  If  the  period  of  cooling  is 
relatively  short,  the  crystals  will  be  small  in  size,  some- 
times only  perceptible  with  a  microscope;  if  it  is  slower, 
the  crystals  grow  till  they  are  very  easily  visible  to  the 
naked  eye. 

How  does  one  study  an  igneous  rock,  to  distinguish  it  from 

sedimentary  and  metamorphic  ones? 

When  one  observes  such  a  rock,  he  observes  its  compact 
texture  (one  crystal  fitting  into  another  without  loss  of 
space);  he  notes  the  straight  sharp  geometric  outlines  of 
some  of  the  crystals  (whose  forms  have  not  been  interfered 
with  by  intergrowths  with  neighboring  crystals),  in 
distinction  to  the  rounded  or  broken  outlines  of  the  grains 
of  the  sedimentary  rock.  He  observes,  also,  that  in 
general  the  more  prominent  crystals  do  not  have  any 
common  direction  of  elongation;  that  is,  they  are  not 
parallel  with  one  another,  as  are  usually  the  crystals  of 
metamorphic  rocks,  but  lie  in  all  possible  positions. 

Can  one  always  readily  distinguish  igneous  rocks  by  this  test? 
There  are  exceptions  to  this  rule,  and  in  the  field  the 

*  Arnold  Hague,  'Geological  Excursion  to  the  Rocky  Mountains,'  p.  350. 


82  GEOLOGY  APPLIED   TO   MINIKG. 

observer  will  meet  difficult  cases.  In  some  hardened  rocks 
where  the  individual  grains  are  not  visible  to  the  naked  eye, 
it  may  be  hard  to  decide  as  to  the  origin,  without  a  micro- 
scope. There  are,  also,  cases  of  igneous  rocks  where  there 
is  a  general  parallelism  of  the  crystals,  though  never  on  so 
complete  a  scale  as  in  metamorphic  rocks.  In  the  cases 
referred  to,  the  crystals,  originally  not  parallel,  have  been 
so  arranged  by  flowage. 

THE  DIFFERENT  KINDS  OF  IGNEOUS  ROCKS. 

How  are  igneous  rocks  classified? 

Igneous  rocks  have  been  named  in  various  ways,  accord- 
ing to  various  classifications.  All  these  classifications  are 
more  or  less  artificial,  and  each  worker  must  select  the  one 
which  serves  his  purpose  best.  Various  features  have 
served  as  main  distinctions — chemical  composition,  mineral 
composition,  form,  structure,  geologic  age,  mode  of  occur- 
rence, locality,  etc.  For  one  purpose  a  chemical  classifi- 
cation may  be  best,  for  another  a  classification  as  to  mode 
of  occurrence,  etc.  The  commonly  accepted  classification 
is  based  partly  on  mineral  composition,  partly  on  chemical 
composition,  and  partly  on  structure. 

How  are  the  different  kinds  of  igneous  rocks  studied  and 

identified? 

Since  the  adaptation  of  the  microscope  to  petrographic 
work,  the  science  has  been  revolutionized;  and  now  all  the 
real  study  is  done  with  that  instrument.  As  a  result  of  the 
detailed  investigation  thus  made  possible,  the  distinctions 


IGNEOUS     KOCKS.  83 

have  in  many  cases  been  finely  drawn,  and  the  number  of 
rock  names  has  rapidly  multiplied. 

7s  it  an  easy  thing  to  name  an  igneous  rock  correctly  in  the 

field? 

Petrologists  very  commonly  have  difficulty  in  defining 
a  given  igneous  rock,  without  resource  to  their  microscope. 
To  take  lava-rocks,  for  example,  one  cannot  always  decide 
without  the  microscope  between  many  varieties  of  rhyolite, 
phonolite,  trachyte  and  andesite.  The  same  is  true  of  the 
dike  rocks,  which  have  received  a  great  number  of  special 
names,  now  falling  into  disuse.  In  coarse-grained  rocks, 
the  petrologist  can  be  more  certain,  yet  it  sometimes 
becomes  difficult  in  the  field  to  distinguish  certain  kinds  of 
granites,  syenites  and  diorites  from  one  another,  and  to 
decide  between  some  diorites  and  diabases. 


CLASSIFICATION  OF  IGNEOUS  ROCKS  FOR  MINING 

MEN. 

To  what  extent  is  it  possible  and  necessary  for  the  miner  to 

classify  igneous  rocks? 

When  such  is  the  case,  it  is  plain  that  the  fine  distinctions 
of  rock  species  are  beyond  the  mining  engineer  and  the 
miner.  Luckily,  such  distinctions,  even  could  he  make 
them,  would  be  of  slight  use  to  him.  But  broad  demarca- 
tions are  necessary,  and  may  be  made  upon  physical 
characteristics,  without  the  microscope  and  chemical 
analysis,  and  with  only  a  slight  knowledge  of  mineralogy. 


84  GEOLOGY  APPLIED  TO  MINING. 

On  what  principles  is  such  a  practical  classification  based? 

The  classification  the  writer  offers  for  this  purpose  is 
based  on:  (1)  Structure.  (2)  Mineralogical  composition. 
Rocks  are  first  divided  into  granular,  coarse  porphyritic, 
fine  porphyritic,  and  glassy  forms. 

What  is  meant  by  a  granular  igneous  rock? 

The  term  granular  is  applied  to  a  fairly  even  texture,  the 
constituent  minerals  being  of  nearly  uniform  size,  and 
generally  interlocking. 

What  is  meant  by  a  porphyritic  igneous  rock? 

Porphyritic  rocks  do  not  have  their  constituent  minerals 
of  uniform  size.  There  is  a  fine  grained  portion,  which 
may  be  dense  and  show  no  crystals  to  the  naked  eye,  or 
may  sometimes  be  non-crystalline,  like  glass.  This  is  the 
groundmass.  Through  it  are  sprinkled  crystals  of  larger 
size,  generally  with  perfect  geometric  outline,  and  often 
separated  from  one  another,  so  as  to  be  completely  sur- 
rounded by  the  groundmass.  These  are  porphyritic 
crystals  or  phenocrysts,  and  the  rock  possesses  porphyritic 
structure. 

In  the  coarse  porphyritic  structure,  the  groundmass  is 
crystalline,  the  individual  minerals  in  it  are  usually  visible 
to  the  naked  eye,  or  by  the  aid  of  a  hand-lens,  and  the 
porphyritic  crystals  are  correspondingly  large.  In  the 
fine,  porphyritic  structure,  the  groundmass  is  fine,  the 
individual  grains  being  difficultly  or  not  at  all  discernible 
to  the  naked  eye;  or  it  may  be  glassy. 


IGNEOUS     ROCKS.  85 

What  is  meant  by  a  glassy  igneous  rock? 

Glassy  rocks  have  no  or  few  porphyritic  crystals;  neither 
do  they  show  any  grains,  even  under  the  microscope — they 
are  smooth  and  homogeneous,  like  glass. 

What  is  the  relative  abundance  of  these  different  kinds  of 

rock? 

Granular  rocks,  coarse  porphyritic  and  fine  porphyritic 
rocks  are  common;  wholly  glassy  rocks  are  relatively 
rare,  and  are  found  chiefly  among  the  outpourings  of 
volcanoes. 

CLASSIFICATION  OP  IGNEOUS  ROCKS. 

A.  GRANULAR  ROCKS  .  Relatively  coarse ;  crystals  of  con- 
stituent minerals  easily  visible  to  the  naked  eye,  and  all 
of  about  the  same  size. 

1.  Granitic   Rocks.     Color  gray,   reddish   or  greenish. 
Relatively  light  in   color  and  weight.    Quartz  abundant, 
while  dark  minerals    (hornblende,   mica,   pyroxene,  etc.) 
form  only  a  small  portion  of  the  rock.    Mica  apt  to  be 
more  abundant  than  in  other  granular  rocks.     Forms  of 
mineral  grains  in   general   short   and  blunt.     Chief  con- 
stituent minerals,  quartz,  feldspar,  mica,  hornblende. 

2.  Dioritic  Rocks.     Of  medium  dark  color  and  medium 
weight;  mottled,  generally  green,  rocks.     Quartz  scarce 
or  absent;   dark  minerals   (especially  hornblende)  fairly 
abundant.    Mica  may  be  present,  but  is  generally  less  in 
amount    than    other   dark    minerals.       Pyroxene    may 
occur.    Grains  of  individual  minerals  have  a  tendency  to 
elongated  forms,  though  they  may  be  short.      Constit- 
uent minerals,  feldspar,  hornblende,  mica. 

3.  Diabasic  Rocks.      Very   dark  and  heavy,  green  of 


86  GEOLOGY  APPLIED  TO  MINING. 

various  shades,  often  black.  No  quartz,  and  very  large 
proportion  of  dark  minerals.  Mica  almost  always  absent. 
Pyroxene  is  usually  very  abundant,  and  there  is  often 
oli vine  and  hornblende.  Magnetite  in  small  grains  (us- 
ually invisible  to  the  naked  eye)  is  nearly  always  present. 
Crystal  forms  generally  elongated.  Chief  constituent  min- 
erals, feldspar,  pyroxene,  olivine. 

4.  Peridotitic  Rocks.  Color,  very  dark  green  or  black, 
darker  and  heavier  than  any  of  the  foregoing.  Are  dis- 
tinguished by  the  absence  of  feldspar.  Often  contain  con- 
siderable quantities  of  the  metallic  minerals  (such  as 
magnetite,  pyrrhotite,  ilmenite,  etc.)  in  small  grains. 
Chief  constituent  minerals,  olivine,  pyroxene,  and  horn- 
blende. Any  one  of  these,  or  any  two,  may  in  some  cases 
be  entirely  lacking,  leaving  the  rock  composed  essentially 
of  two  of  the  above-named  minerals,  or  even  one. 

B  COARSE  PORPHYRITIC  ROCKS.  Are  spotted  with 
well-formed  crystals  of  the  common  rock-forming  min- 
erals, quartz,  feldspar,  mica,  hornblende,  pyroxene,  etc., 
which  are  contained  in  a  groundmass  composed  of 
interlocking  crystals  of  markedly  smaller  size  than  the  por- 
phyritic  crystals. 

1.  Granitic  Porphyry.     Combines  the  coarse  porphyritic 
structure  with  the  same  physical  and  mineralogical  char- 
acters as   the   granitic    rocks,    as    denned  above.     Chief 
constituent  minerals,  quartz,  feldspar,  mica,  hornblende. 

2.  Dioritic  Porphyry.     Combines  the  coarse  porphyritic 
structure  with  the  same  physical  and  mineralogical  char- 
acters as  the  dioritic  rocks,  as  described   above.     Chief 
constituent  minerals,  feldspar,  hornblende,  mica. 

3.  Diabasic  Porphyry.     Combines  the  coarse  porphyri- 
tic structure   with  the  same  physical  and    mineralogical 
characters  as  diabasic  rocks,   as  described  above.     Chief 
constituent  minerals,   feldspar,  pyroxene,  olivine. 


IGNEOUS     ROCKS.  87 

C.  FINE  PORPHYRITIC  ROCKS.  Like  the  coarse  por- 
phyritic rocks,  but  the  groundmass  is  finer,  so  that  the 
individual  crystalline  grains  in  it  are  barely  or  not  at 
all  visible  to  the  naked  eye. 

1.  Rhyolitic  Rocks.     These  are  chemically  and  miner- 
alogically  the  same  as  the  granitic  rocks  and  the  granitic 
porphyry  rocks,  but  differ  in  having  the    fine    porphyritic 
structure.      Rhyolitic  rocks    are  generally  of    light    color 
(white,  light  gray,  pink,  red,  etc.)  and  of  relatively  light 
weight.     As     porphyritic  crystals     they    generally    show 
quartz,  hexagonal  in  cross-section,  and  frequently  short, 
blunt  feldspar.     Crystals    of   dark  mica    are  usual,  and 
often  also  hornblende;   but  the  amount  of  dark  minerals 
is  relatively  small.     The   groundmass    is  generally  rather 
rough  to  the  touch,  and  looks   and   feels  somewhat    like 
broken  coarse  earthenware;    the    individual  grains  in  it 
are  usually   not   distinguishable.    Chief    constituent    min- 
erals,  quartz,   feldspar,  mica,  hornblende. 

2.  Andesitic  Rocks.     These  are  chemically  and  miner- 
alogically  the  same   as  the  dioritic  rocks  and  the  dioritic 
porphyry  rocks,  but  differ  in  having  the  fine  porphyritic 
structure.      In   color  the  andesitic  rocks   are  dark  gray, 
medium  brown,  dark  red,  etc.    They  are  of  medium  weight. 
Quartz  is  usually  not  found  as  porphyritic  crystals,  and 
mica  is  not  as  common  as  in  rhyolitic  rocks.     The  por- 
phyritic crystals  are  most  apt  to  be   feldspar   and  horn- 
blende, often   pyroxene.     Dark   minerals    in    general  are 
rather  abundant.      Groundmass  generally  slightly  coarser 
than   with    the    rhyolitic     rocks;    the  individual   grains, 
though  they  may  be  tiny,   are  often  visible  either  to  the 
naked    eye    or  through   a   hand-lens.     Chief  constituent 
minerals,  feldspar,  hornblende,  pyroxene,  mica. 

3.  Basaltic  Rocks.     These   are  chemically  and  miner 
alogically  the  same  as  the  diabasic  rocks,  and  the  diabasic 


88  GEOLOGY  APPLIED  TO  MINING. 

porphyry  rocks,  but  differ  from  them  in  having  the  fine 
porphyritic  structure.  The  porphyritic  crystals  are  gen- 
erally few,  and  do  not  differ  so  markedly  in  size  from 
the  groundmass  crystals  as  in  the  rhyolitic  rocks  and  the 
andesitic  rocks.  The  groundmass  is  generally  coarser 
than  in  the  andesitic  and  rhyolitic  rocks;  the  individual 
grains  in  it,  though  fine,  can  often  be  seen  by  the  naked 
eye.  If  they  cannot,  there  are  very  likely  no  porphyri- 
tic crystals  to  be  seen.  Basalts  contain  as  a  rule  no 
quartz  or  mica.  They  are  usually  black  in  color  and 
heavy.  Where  minerals  can  be  distinguished  in  them, 
they  are  usually  pyroxene,  feldspar,  or  olivine.  Chief 
constituent  minerals,  feldspar,  pyroxene,  olivine. 


ADDITIONAL  DEFINITIONS. 

Does  this  classification  embrace  all  the  rock  names  necessary 

to  a  miner? 

In  the  writer's  opinion,  this  is  about  as  far  as  one  who  is 
not  a  petrologist  can  safely  go.  There  are,  however,  other 
rock  names  frequently  used  by  miners,  on  which  observa- 
tions will  be  made. 

What  is  quartz  porphyry? 

This  is  familiar  to  mining  men  as  one  of  the  most  impor- 
tant rocks  in  Leadville  and  other  mining  regions.  The 
name,  formerly  in  good  use  by  geologists,  is  being  dropped 
for  granite  porphyry  or  rhyolite  porphyry,  the  former  for 
the  coarse  grained,  the  latter  for  the  finer  grained  varieties. 
The  description  of  granite  porphyry  is  that  of  quartz  por- 
phyry. 


IGNEOUS     ROCKS.  89 

What  is  syenite? 

This  is  a  favorite  term  with  miners.  A  syenite  is  a 
granite  without  quartz.  Like  granite,  it  has  a  light  color 
and  relatively  light  weight,  contains  relatively  small 
amounts  of  the  dark-colored  minerals,  and  is  apt  to  con- 
tain mica.  It  is  not  always  easy  to  distinguish  syenite 
from  diorite  in  hand  specimens.  Syenites  are  compara- 
tively rare  rocks. 

What  is  trachyte? 

Trachyte  was  formerly  a  much-used  term  with  geologists. 
In  the  Great  Basin  of  Nevada,  for  example,  enormous 
quantities  of  volcanic  rocks  were  classified  as  trachyte. 
Now  microscopic  study  has  shown  them  to  be  mainly  ande- 
sites,  and  that  none  of  them  are  trachytes.  Trachyte  is  still 
a  popular  term  with  miners,  but  now  we  know  it  to  be  a 
comparatively  rare  rock.  True  trachyte  bears  the  same 
relation  to  syenite  as  rhyolite  does  to  granite;  it  has  the 
chemical  and  mineralogical  composition  of  syenite,  but 
with  the  fine  porphyritic  structure.  It  is,  therefore,  a 
rhyolite  without  quartz. 

What  is  phonolitc? 

Since  this  rock  occurs  in  connection  with  the  famous 
gold  ores  of  Cripple  Creek,  in  Colorado,  it  has  become  well 
known  in  the  mining  world.  It  is  often  difficult  to  identify 
phonolite  without  microscopical  or  chemical  tests.  Phono- 
lite  contains,  besides  feldspar,  nepheline,  leucite,  or  both, 
and  pyroxene,  sometimes  hornblende.  The  colors  are 


90  GEOLOGY  APPLIED  TO  MINING. 

usually  gray  or  green.    Phonolites  are  also  relatively  rare 
rocks. 

It  is  popularly  supposed  that  the  metallic  ring  emitted 
by  fragments  of  some  volcanic  rocks  is  a  test  for  phonolite; 
but  this  is  an  antiquated  idea.  Rhyolites  and  other  rocks 
frequently  give  this  ring. 

What  is  amygdaloid? 

Amygdaloid  is  a  term  applied  to  lavas  which  are  cellular, 
that  is,  are  full  of  little  holes  or  amygdules,  which  were 
filled  by  steam  at  the  time  of  consolidation.  In  the  Lake 
Superior  region  certain  amygdaloidal  basalts  have  their 
amygdules  filled  with  native  copper,  and  so  become  ores 
and  of  considerable  interest  to  the  miner. 

What  is  doleritcf 

Dolerite  is  a  term  that  has  been  used,  sometimes  instead 
of  diabase,  sometimes  instead  of  basalt. 

What  is  gdbbro? 

The  term  gabbro  is  applied  to  certain  granular  rocks  con- 
sisting chiefly  of  feldspar  and  pyroxene.  It  thus  falls 
within  the  group  of  diabasic  rocks,  in  the  foregoing  classi- 
fication. 

What  is  felsite? 

Felsite  is  a  general  term  applied  to  certain  light-colored, 
very  fine-grained  igneous  rocks,  chiefly  altered  rhyolites. 
Strictly  speaking,  felsite  is  hardly  an  accurate  term,  and 
most  rocks  so  called  may  be  proved  to  be  rhyolite  or 


IGNEOUS     ROCKS.  91 

rhyolite  porphyry.  In  felsites  the  porphyritic  crystals  are 
small  and  few,  or  have  become  inconspicuous  on  account 
of  decomposition,  and  so  are  not  visible  to  the  naked  eye. 
The  term  is  an  allowable  one,  and,  on  account  of  its  broad 
definition,  not  difficult  of  application. 

Example:  Study  of  the  felsites  of  Carodoc,  Wales,  show 
microscopic  structures  which  had  been  altered  almost 
beyond  recognition,  but  which  indicate  that  these  rocks 
were  originally  partly  rhyolites  and  partly  sedimentary 
beds  derived  from  the  erosion  of  rhyolites  (rhyolite  tuffs).* 

What  is  greenstone? 

Greenstone  is  a  general  name  applied  to  certain  igneous 
rocks,  generally  rather  fine-grained,  of  a  general  dark 
green  color.  The  term  is  used  by  geologists,  especially 
when  no  more  accurate  definition  is  possible  in  the  field. 
The  greenstones  are  usually  old  rocks  geologically,  and  the 
green  color  is  the  result  of  thorough  alteration.  They  are 
diabases  or  diorites,  sometimes  old  andesites  and  basalts. 
The  term  is  admissible  and  of  easy  application. 

Example:  In  northeastern  Minnesota,  on  the  eastern 
part  of  the  Mesabi  iron  range,  are  rocks  which  have  been 
called  greenstones  because  of  their  general  dark  greenish 
color.  They  are  crystalline  rocks  composed  usually  of 
hornblende  and  feldspar.  Mineralogically,  the  rocks  are 
diorites,  but  they  have  been  recrystallized  from  other 
rocks,  some  of  which  were  certainly  diabases  and  andesites 
and  some  probably  diorites  or  gabbros.f 

*  F.  Rutley,  Quarterly  Journal,  Geological  Society,  Vol.  XLVII,  p.  512. 
f  U.  S.  Grant,  'Engineer's  Year  Book,'  University  of  Minnesota,  1898,  p.  54. 


92  GEOLOGY  APPLIED  TO  MINING. 

What  is  pegmatite  or  giant  granite? 

Pegmatite  or  giant  granite  is  a  name  applied  to  those 
common  dikes,  generally  granitic,  where  the  grain  is  ex- 
ceedingly coarse,  individual  crystals  being  frequently 
several  inches  across. 

What  is  serpentine  rock? 

This  is  a  rock  consisting  partly  or  wholly  of  the  dark- 
green,  greasy-feeling  mineral  serpentine.  It  is  a  metamor- 
phic  or  altered  rock,  and  in  many  cases  is  derived  from 
igneous,  chiefly  peridotitic  rocks.  The  decomposition  of 
the  olivine  and  pyroxene  of  peridotites  usually  affords 
serpentine. 

What  is  trap? 

Trap  is  an  old  general  name  for  dense,  dark-colored  dike 
rocks.  The  term  is  still  in  use.  A  trap  dike,  more  accu- 
rately considered,  may  be  made  up  of  andesite,  basalt, 
diorite  or  diabase,  etc. 

What  is  breccia?  * 

Breccia  is  an  Italian  word,  applied  to  crushed  and  broken, 
yet  still  consolidated  rock.  A  breccia  resembles  a  con- 
glomerate in  being  composed  of  coarse  fragments  packed 
together;  but  in  the  former  the  pieces  are  sharp  and  angular, 
in  the  latter  rounded  by  water  action,  indicating  their 
origin.  It  is  generally  easy  to  recognize  a  breccia,  but 
sometimes  it  is  difficult  to  tell  how  it  originated.  Where 
there  has  been  movement  in  a  rock,  as  along  the  vicinity  of 

*  Pronounced  bre"k-she-ah,  with  acg^nt  9n  the  first  syllable. 


IGNEOUS    ROCKS.  93 

a  fault,  a  breccia  is  developed,  called  a  friction  breccia. 
Lava,  on  the  other  hand,  is  often  shattered  by  explosion 
attending  its  eruption  or  by  being  forced  into  renewed 
movement  when  partially  hardened.  The  result  is  a 
volcanic  breccia. 

• 
What  is  pumice? 

Pumice  is  a  glassy  lava,  which  at  the  time  of  hardening 
was  so  full  of  steam-filled  cavities  that  it  now  has  a  spongy 
structure,  and  is  so  light  that  it  often  floats :  it  is  a  sort  of 
lava  froth. 

TRANSITIONS  BETWEEN  DIFFERENT  KINDS  OF 
IGNEOUS  ROCKS. 

Are  the  divisions  of  igneous  rocks,  as  given,  sharply  divided 
from  one  another  in  point  of  mineralogical  composition? 
Igneous  rocks  form  a  connected  series,  with  gradual 
transitions  from  one  of  the  artificial  divisions  above  out- 
lined to  the  other.  This  is  the  case  in  any  classification. 
No  matter  how  many  divisions  are  made,  some  rocks 
will  be  found  occupying  the  border  lines.  So  it  is  very 
possible,  for  example,  to  classify  a  rock  in  the  field  as  a 
diorite,  which  closer  study  would  show  to  be  a  diabase. 
The  suffix  "ic,"  as  diorit-ic,  in  the  foregoing  scheme,  ex^ 
presses  a  provisional  determination,  and  saves  one  from 
the  charge  of  making  hasty  and  faulty  decisions. 

Are  the  different  igneous  rocks  separated  sharply  in  point  of 
texture? 
In  point  of  texture  there  are  all  transitions  between 


94  GEOLOGY  APPLIED  TO  MINING. 

granitic  rocks,  granitic  porphyry  rocks,  and  rhyolitic  rocks; 
also  between  dioritic  rocks,  dioritic  porphyry  rocks  and 
andesitic  rocks;  and,  again,  between  diabasic  rocks, 
diabasic  porphyry  rocks  and  basaltic  rocks;  in  short, 
between  granular  rocks,  coarse  porphyritic  rocks,  and  fine 
porphyritic  rocks.  . 

How  did  these  different  textures  originate? 

The  difference  in  general  seems  to  depend  on  the  rapidity 
with  which  the  rock  cooled,  those  which  cooled  more 
slowly  having  had  more  time  to  crystallize,  and  hence  pro- 
ducing larger  crystals  and  coarser  rock  texture.  The 
rapidity  of  cooling  depends  in  large  part  on  proximity  to 
the  surface.  Those  which  cool  at  the  surface  chill  quickly; 
while  those  that  harden  deep  in'  the  earth's  crust  retain 
their  heat  for  a  long  time.  Therefore  it  is  that  the  lavas 
or  surface  rocks  are  almost  wholly  of  fine  porphyritic 
structure.  Small  masses  of  molten  rock  thrust  into  older 
harder  rocks  and  there  cooling  (dikes),  generally  have  the 
coarse  porphyritic  structure,  though  often  the  fine  porphy- 
ritic. Large  masses  of  rock  cooling  at  a  distance  from  the 
earth's  surface  have  generally  the  granular  structure. 
Cases  may  occur  where  a  rock  mass  may  be  fine  porphyritic 
on  the  edges,  where  it  cooled  quickly,  coarse  porphyritic 
further  in,  and  granular  in  the  center,  but  in  general  rocks 
are  more  or  less  homogeneous. 

Are  the  different  igneous  rocks  sharply  separated  in  point  of 
composition? 
Similarly,  a  single  rock  mass  may  vary  in  composition,  so 


IGNEOUS     ROCKS.  95 

that,  for  example,  it  is  a  diabase  on  the  borders  and  a 
diorite  in  the  center;  yet  usually  the  different  types  of  rock 
are  distinct  in  the  field. 

Example:  The  exceptional  occurrence  of  different  rock- 
types  as  part  of  a  single  dike  is  shown  at  a  locality  in 
Michigan,  near  Crystal  Falls.  Here,  there  has  been 
observed  a  dike,  four  feet  wide,  which  cuts  a  mass  of 
gabbro.  Near  the  center  of  the  dike  the  rock  is  a  granite, 
containing  biotite,  while  the  sides  consist  of  diorite  without 
any  quartz.  The  two  rocks  are  supposed  to  have  separated 
one  from  another  while  the  mass  was  in  a  molten  state.* 


FORMS  OF  IGNEOUS  ROCKS. 

In  what  different  forms  do  masses  of  igneous  rocks  occur? 

The  forms  in  which  igneous  rocks  occur  may  be  outlined 
thus: 

1.  Fundamental. 

2.  Intrusive      (Masses,  dikes,  sills  or  sheets.) 

3.  Extrusive.     (Lavas.) 

What  are  the  fundamental  igneous  rocks  and  what  is  their 

origin? 

The  fundamental  igneous  rocks  underlie  the  oldest 
stratified  rocks.  They  are  mostly  Archaean  granites  and 
form  the  floor  on  which  the  first  recognizable  sediments 
were  laid  down.  Many  of  them  may  be  metamorphosed 
and  crystallized  early  sediments. 

*  J.  M.  Clements,  Journal  Geology,  Vol.  IV,  No.  4,  p.  377. 


96  GEOLOGY  APPLIED  TO  MINING. 

What  are  intrusive  rocks? 

In  the  molten  rocks,  which  exist  below  or  within  the 
crust*  of  the  earth,  important  movements  and  migrations 
occur.  The  plastic  material  is  propelled  upward  by  the 
steam  which  it  contains  and  by  other  causes,  and  so  forces 
its  way  into  and  through  the  hard  rocks  nearer  the  surface. 
It  enters  these  rocks  along  the  line  of  least  resistance — 
along  fissures,  fault-planes,  joints,  crushed  zones,  or 
bedding  planes. 

What  is  an  intrusive  mass? 

The  molten  rock  may  come  up  in  large  volume,  thrusting 
aside  or  absorbing  the  rocks  it  enters  (the  intruded  rock), 
and  thus  forming  a  mass  of  irregular  shape. 

What  is  a  dike  of  igneous  rockf 

Where  the  molten  rock  ascends  along  fissures  or  other 
similar  channels,  it  will  form  a  zone,  or  a  body  with  very 
slight  thickness  as  compared  with  its  extent  in  other 
directions.  In  form,  a  dike  has  the  same  characters  as  a 
vein.  Its  boundaries  are  generally  straight  planes,  and  it 
may  dip  away  from  the  horizontal  at  all  possible  angles.  A 
dike  may  be  an  inch  or  a  mile  across,  but  it  is  usually 
vastly  longer  than  it  is  thick.  (Fig.  6.)  From  a  great 
mass  of  intrusive  rock  there  are  usually  smaller  dikes 
which  run  out  into  the  surrounding  formations. 

What  are  sheets  or  sills  of  intrusive  rockf 

In  a  sedimentary  rock,  when  the  dikes  run  along  the 
bedding  planes,  and  so  are  parallel,  or  nearly  so,  to  the 


IGNEOUS     ROCKS. 


97 


98  GEOLOGY  APPLIED  TO  MINING. 

stratification,  they  are  called  sheets  or  sills.  A  sheet  or 
sill  may  be  thick  or  thin;  there  sometimes  may  be  many 
of  them,  alternating  with  beds  of  sedimentary  rock. 

How  is  it  that  we  find  fundamental  and  intrusive  igneous 

rocks  at  the  earth's  surface? 

Although  fundamental  and  intrusive  rocks  were  originally 
far  below  the  surface,  yet  by  the  long  process  of  erosion 
the  overlying  mass  has  been  stripped  off  and  these  rocks  are 
exposed  to  the  light  of  day.  That  is  why  we  now  find 
them  at  the  surface  as  often  as  we  do  those  which  were 
poured  out  of  volcanoes. 

What  are  extrusive  rocks  or  lavas? 

Extrusive  rocks  are  those  which  reach  the  surface  by 
means  of  the  conduits  above  mentioned,  and  overflow  either 
from  volcanoes,  as  explosive  eruptions,  with  ashes  and 
scoriae,  or  as  quiet  wellings-out,  either  from  volcanoes  or 
fissures.  Beneath  the  surface  are  dikes  which  have  been 
the  feeders,  and  the  same  lava  may  have  spread  out  along 
the  bedding  planes  of  the  stratified  rocks  as  sheets  or  sills. 

Example:  Examples  of  the  pouring  out  of  lavas  from 
volcanoes,  with  attendant  showers  of  ash,  pumice,  etc.,  are 
too  well  known  to  require  specific  mention. 

An  instance  of  the  quiet  welling  out  of  a  great  mass  of 
lava  along  fissures  is  furnished  by  the  Columbia  river  basalt, 
which  covers  a  great  arid  plain  in  Idaho,  Oregon  and 
Washington.  This  lava  consists  of  a  series  of  flows  from 
20  to  150  feet  thick,  piled  one  on  top  of  another.  The 
dikes  which  were  the  feeders  to  the  flows  are  now  in  part 
exposed  in  Oregon,  by  the  erosion  of  the  overlying  rock; 


IGNEOUS     ROCKS.  99 

and  the  lack  of  any  ash  or  fragmental  material,  or  volcanic 
cones,  shows  that  the  eruption  was  quiet.*  In  Idaho  a 
total  thickness  of  as  much  as  5,000  feet  of  this  basalt  is 
shown  by  the  Snake  River  canon,  which  has  cut  down 
through  it.  Before  the  eruption  of  the  lava  there  was  in 
this  region  a  varied  topography,  but  the  mountain  ranges, 
deep  valleys,  and  canons  were  all  blotted  out  by  the  swiftly 
succeeding  flows,  until  only  the  very  highest  peaks  still 
show  their  heads,  f 

GENERAL  RELATION  BETWEEN  IGNEOUS  ROCKS 
AND  ORE-DEPOSITS. 

Is  there  any  relation  between  igneous  rocks  and  ore-deposits? 
There  is  the  very  closest  connection.  Probably  nine  out 
of  ten  ore-deposits  have  some  visible  relation  to  a  body 
of  igneous  rocks.  In  general,  also,  a  country  free  from 
igneous  rocks  has  a  scarcity  of  Ore-deposits. 

What  is  the  reason  for  this  relation? 

The  reasons  for  this  are,  in  part  at  least,  known.  All 
igneous  rocks  contain  metals.  Iron,  for  example,  is  present 
in  every  igneous  rock  in  large  amount — the  percentage  of 
the  whole  rock  being  in  some  diabases  and  basalts  15  or  20 
per  cent.,  or  even  more.  Manganese,  also,  is  present  in 
most  igneous  rocks  in  noticeable  quantities.  When  we 
come  to  the  rarer  metals,  we  naturally  do  not  find  them  in 
such  amounts;  but  chemists  have  proved  the  presence  of 

*  W.  Lindgren,  22d  Annual  Report  United  States  Geological  Survey,  Part  II, 
p.  741. 

t  W.  Lindgren,  20th  Annual  Report  United  States  Geological  Survey,  Part 
III,  pp.  91,  93. 


100          GEOLOGY  APPLIED  TO  MINING. 

such  metals  as  copper,  lead,  zinc,  nickel,  tin,  etc.,  in  nearly 
every  kind  of  igneous  rock.  These  metals  usually  occur  as 
constituents  of  the  dark-colored  silicates  (hornblende,, 
pyroxene,  dark  mica,  olivine,  etc.)  Even  the  rarest,  such  as 
silver,  platinum  and  gold,  are  similarly  found,  though  in 
small  quantities. 

Do  metals  occur  in  igneous  rocks  except  as  constituents  of  the 
dark-colored  silicates? 

The  commoner  metals  occur  in  igneous  rocks  in  the  same 
form  that  they  do  in  ore-deposits — in  the  form  of  sulphides 
and  oxides.  This  has  been  proved  in  the  case  of  iron  pyrite, 
the  magnetic  sulphide  of  iron  (pyrrhotite),  the  magnetic 
oxide  of  iron  (magnetite),  the  magnetic  oxide  of  iron  with 
titanium  (ilmenite  or  titanic  iron),  the  oxide  of  iron  and 
chromium  (chromite),  etc. 

Example:  In  the  diabase  rocks  of  the  Grass  Valley  dis- 
trict, California,  there  are  small,  very  abundant  grains  of 
pyrite,  pyrrhotite,  and  ilmenite,  which  occur  within  the 
augite  and  feldspar  of  the  rocks  in  such  a  way  as  to  prove 
that  the  metallic  minerals  are  of  primary  origin  (that  is, 
that  they  have  crystallized  out  of  the  cooling  molten  rock, 
and  have  not  been  introduced  subsequent  to  its  consolida- 
tion), and  indeed  were  the  first  of  the  rock-forming  minerals 
to  solidify.  (Fig.  7.) 

Do  not   the  sedimentary  rocks   also   contain  disseminated 
metals? 

Chemical  investigation  seems  to  bear  out  the  statement 
that  as  a  rule  the  sedimentary  rocks,  although  they  also 


IGNEOUS     tfQCJKS.,  101 

contain  small  quantities  of  the  metals,  yet  are  relatively 
poorer  in  these  than  the  igneous  rocks. 

Example:  Mr.  Luther  Wagoner*  has  made  a  series  of 
delicate  determinations  of  gold  and  silver  in  certain  igneous 
and  sedimentary  rocks  of  California.  Four  specimens  of 
granite  showed  respectively  the  following  weights  in  milli- 
grams per  ton:  gold,  104,  137,  115,  1130;  silver,  7660,  1220; 


Fig.  7.  Primary  pyrrhotite  in  augite.     Black,  pyrrhotite;  a.  augite;  b.  uralite;t 

c.  chlorite.     From  W.  Lindgren;  17th  Annual  Report  United 

States  Geological  Survey,  Part  II. 

940;  5590.  One  specimen  of  syenite  contained  gold,  720; 
silver,  15,430.  A  specimen  of  diabase  contained  gold,  76; 
silver,  7,440.  One  of  the  basalt  gave  gold,  26;  silver,  547. 
The  sedimentary  rocks  tested  were  three  specimens  of 
sandstone  and  two  of  marble  (one  of  the  latter  from  Italy). 
The  sandstones  gave  respectively  in  gold,  39,  24,  and  21;  in 

*  'Detection  and  Estimation  of  Gold  and  Silver.'     Transactions  American 
Institute  Mining  Engineers,  Vol.  XXX,  p.  798. 

*  A  variety  of  hornblende. 


102  C?EOLOGY   APPLIED    TO    MINING. 

silver,  540,  450,  320.  The  California  marble  showed  gold, 
5;  silver,  212;  the  Italian  sample,  gold,  8.63;  silver,  201. 
Several  assays  in  San  Francisco  Bay  mud  (containing  some 
organic  material)  gave  gold,  from  45  to  125.  Two  assays 
of  sea- water  gave  a  mean  gold  11.1 ;  silver,  169.5  milligrams 
per  ton. 

On  the  average,  therefore,  the  granite  contained  371 
milligrams  gold  and  3852  silver;  while,  as  before  stated,  the 
syenite  contained  720  gold  and  15,430  silver;  the  diabase 
76  gold  and  7440  silver;  and  the  basalt  26  gold  and  547 
silver.  Taking  the  sedimentary  rocks,  the  sandstones 
averaged  28  gold  and  437  silver;  the  marble,  7  gold  and  206 
silver;  and  the  bay  mud  85  gold. 

Averaging  the  igneous  rocks  assayed,  we  find  a  mean  of 
gold  330  and  silver  5547,  while  the  mean  of  the  sedimentary 
rocks  (sandstones  and  marbles)  is  17  gold  and  344  silver. 
That  is  to  say,  the  mean  of  the  igneous  rocks  assayed  con- 
tains about  19  times  as  much  gold  and  16  times  as  much 
silver  as  the  mean  of  the  sedimentary  rocks. 

Are  disseminated  metals  equally  abundant  in  different  kinds 

of  sedimentary  rocks? 

In  regard  to  the  sedimentary  rocks,  it  will  be  noticed 
that  the  sandstones  in  these  tests  contained  on  an  average 
four  times  as  much  gold,  and  over  twice  as  much  silver  as 
the  marbles;  while  the  bay  mud  (which  in  hardening  would 
become  shale)  contained  nearly  13  times  as  much  as  the 
marble. 

Are  disseminated  metals  equally  abundant  in  different  kinds 

of  igneous  rocks? 

As  regards  the  igneous  rocks,  if  we  take  the  silicious  or 
acid  rocks  (granite  and'-syenite)  thus  examined  and  compare 


IGNEOUS     ROCKS.  103 

their  mean  with  that  given  of  the  basic  rocks  (diabase  and 
basalt),  we  find  that  the  silicious  rocks  showed  nearly  nine 
times  as  much  gold  and  about  one  and  a  half  times  as  much 
silver  as  the  basic  ones.  It  remains  to  be  seen  whether 
further  data  would  confirm  these  results. 

What  bearing  has  the  presence  of  disseminated  metals  in 
igneous  rocks  on  the  question  of  the  relation  between  igneous 
rocks  and  ore-deposits? 
The  metals  disseminated  in  igneous  rocks  often  become 

-concentrated  by  various  agencies. 

7s  there  no  other  reason  for  this  relation? 

There  is  another  reason,  and  one  perhaps  as  important, 
why  ore-deposits  are  so  closely  connected  with  igneous 
rocks.  These  rocks  retain  their  heat  a  long  time  before 
cooling.  In  the  case  of  rocks  that  cool  beneath  the  earth's 
surface,  it  is  safe  to  say  that  they  keep  their  heat  for  cen- 
turies of  centuries.  So  all  circulating  waters  passing 
through  them  become  heated,  and  the  hot  water  having 
less  specific  gravity  than  cold  water  (water  expands  on 
heating)  has  a  tendency  to  rise,  and  to  appear  at  the  surface 
as  hot  springs.  This  hot  water  has  a  power  of  solution — 
and  hence  a  power  of  concentrating  the  disseminated 
metals  (whether  in  the  igneous  or  in  the  stratified  rocks) 
into  ore-deposits — many  times  greater  than  cold  waters. 
There  are  three  phenomena  frequently  found  to  be  con- 
nected :  igneous  rocks,  hot  springs  and  ore-deposits.  Often, 
however,  we  find  districts  where  the  igneous  rocks  have 


104          GEOLOGY  APPLIED  TO  MINING. 

long  ago  cooled,  even  far  beneath  the  surface,  and  where 
there  are  ancient  ore-deposits,  but  no  longer  hot  springs. 

Is  the  presence  of  hot  springs  favorable  to  the  finding  of  ore- 


The  presence  of  hot  springs  in  a  country  is  favorable  to 
ore-deposits;  but  their  absence  cannot  be  taken  as  a  sign 
that  such  deposits  do  not  exist.  Hot  springs  are  most 
likely  to  occur  in  regions  of  younger  eruptive  rocks,  (Ter- 
tiary, for  example),  where  the  under  rocks  are  still  hot; 
and  not  so  often  in  the  older  and  perfectly  chilled  rocks. 

How  are  the  disseminated  ores  of  igneous  rocks  concentrated, 

before  or  during  cooling?* 

While  the  rock  is  still  partially  molten  and  fluid,  the 
different  elements  have  some  power  of  moving  about,  and 
it  is  usually  held  that  on  account  of  the  mutual  attraction 
of  like  materials  they  tend  to  group  themselves  and  form 
bodies  more  or  less  concentrated. 

How  is  the  process  of  magmatic  segregation  supposed  to 
effect  the  concentration  of  basic  materials? 

During  the  earlier  period  of  cooling,  the  metallic  sulphides 
and  oxides,  which  are  among  the  first  minerals  to  crystallize, 
and  are  especially  abundant  in  basicf  rocks,  may  collect. 
In  some  igneous  rocks  rich  in  iron,  the  iron  becomes  espe- 
cially abundant  in  places  and  even  may  be  sufficiently 

*  The  material  of  the  following  few  pages  follows  closely  certain  portions  of 
Chapter  I. 

tt.  e.  Dark  colored,  heavy  rocks,  containing  a. low  percentage  of  silica. 


IGNEOUS    ROCKS.  105 

concentrated  to  form  an  ore,  though  always  retaining  its 
character  of  an  original  constituent.  In  Greenland, 
masses  of  native  iron  have  been  found  in  basalt.  Magnet- 
ite, the  magnetic  oxide  of  iron,  is  sometimes  sufficiently 
abundant  to  form  ore-bodies  in  this  way.  Such  is  the 
case,  for  example,  in  Sweden,  in  Rhode  Island,  in  the  Lake 
Superior  region,  in  Canada  and  elsewhere. 

Example:  The  titaniferous  iron  ores  (magnetites)  of  the 
Adirondack  Mountains  in  New  York  are  associated  in  all 
cases  with  basic  igneous  rocks,  which  have  been  intruded 
into  older  gneisses  and  crystalline  limestones.  The  tran- 
sition from  the  basic  wall  rock  (generally  gabbro)  to  ore 
usually  takes  place  gradually,  but  within  a  short  space. 
There  is  no  ore  along  contacts,  nor  any  evidence  of  the 
formation  of  the  ore  after  the  consolidation  of  the  rock. 
The  basic  rock  itself  has  been  split  up  by  segregation  of  its 
essential  minerals,  so  that  some  portions  are  almost  entirely 
of  feldspar,  while  other  portions  contain  large  amounts  of 
pyroxene  (making  a  gabbro) .  It  is  plain  that  the  mag- 
netite which  forms  the  ores  has  been  segregated,  like  the 
feldspar  and  pyroxene,  while  the  rock  was  in  a  partially 
molten  state;  and  it  is  possible  that  the  high  specific  grav- 
ity of  the  iron  may  have  been  influential  in  bringing  about 
this  result.* 

Are  there  other  ore-deposits,  besides  those  of  iron,  which  are 
thus    known    to    be    original,  and  due  to  magmatic  seg- 
regation of  basic  materials  in  igneous  rocks? 
The  mixture  of  oxide  of  chromium  and  oxide  of  iron 

*  J.  F.  Kemp,  19th  Annual  Report  United  States  GeologiqaJ  Survey,  Part 
III,  pp.  383-422. 


106  GEOLOGY  APPLIED  TO  MINING. 

(chromite)  is  frequently  found  in  tiny  crystals  in  the 
igneous  rocks,  and  may  be  so  abundant  in  certain  parts  that 
it  forms  an  ore.  It  may  even  form  solid  masses,  crowding 
out  the  other  rock-forming  minerals.  Many  of  the  known 
chrome  ore-deposits  have  been  held  to  belong  to  this  class. 

Corundum,  the  oxide  of  aluminum,  used  chiefly  as  an 
abrasive  (the  pure  varieties  are  precious  stones — sapphire 
and  ruby),  has,  in  a  number  of  cases,  been  found  to  be  an 
original  constituent  in  igneous  rocks,  not  only  in  small 
quantities,  but  in  those  accumulations  which  are  mined  as 
ores. 

Nickel  is  found  in  fresh  igneous  rocks  as  part  of  a  number 
of  minerals.  Pyrrhotite,  which  is  a  magnetic  sul- 
phide of  iron,  very  commonly  contains  nickel,  and  is  one 
of  the  principal  ores;  it  is  a  not  uncommon  rock-forming 
mineral.  Large  masses  of  nickeliferous  pyrrhotite  have 
been  explained  by  good  authorities  as  original  constituents 
of  igneous  rocks;  but  others  have  questioned  these  con- 
clusions. 

How  does  the  process  of  magmatic  segregation  effect  the  con- 
centration of  silicious  materials? 

During  the  final  stages  of  consolidation,  heated  waters, 
steam  and  gases  (containing  silica,  with  earthy  and  metallic 
minerals,  in  solution),  which  are  left  over  from  the  cooling 
mass,  deposit  their  solid  portions  in  the  form  of  pegmatite 
or  quartz,  as  nests  or  veins  in  the  hardening  rock  or  in 
some  neighboring  formation.  It  is  held  by  some  that 
certain  of  the  quartz  veins  having  this  origin  contain  suf- 


IGNEOUS     ROCKS.  107 

ficient  gold  to  render  them  ores;  but  this  conclusion  is  not 
yet  universally  accepted. 

Do  the  residual  silicious  solutions  always  form  pegmatites  and 

quartz  veins? 

The  residual  silicious  solutions,  instead  of  forming  definite 
veins,  may  penetrate  the  rock  with  which  the  cooling 
igneous  mass  is  in  contact,  and  there  may  deposit  their 
solid  portions,  usually  by  replacement  of  the  original  rock. 

When  the  resulting  altered  rock  contains  ores,  it  is  called 
a  contact  metamorphic  deposit. 

What  are  the  characteristics  of  contact  metamorphic  deposits? 
The  first  mark  identifying  this  class  of  deposits  is  their 
location  at  the  contact  of  an  intrusive  igneous  rock  with 
another  rock,  or  in  evident  close  relation  to  it.  But  this  is 
not  sufficient,  for  other  deposits  may  in  some  cases  be 
formed  along  such  a  contact,  circulating  waters  having 
found  this  the  easiest  channel.  In  the  true  contact  meta- 
morphic deposit,  mineralization  has  been  accomplished  by 
materials  pressed  out  of  cooling  rock.  These  materials 
consist  of  heated  waters  and  aqueous  gas,  mingled  with 
other  gases  of  various  kinds,  and  both  the  escaping  waters 
and  the  gases  may  carry  in  solution  metallic  and  other 
minerals,  which  they  may  deposit  near  the  contact,  in 
concentrated  form.  Under  these  conditions  certain  min- 
erals are  characteristically  formed  which  are  rare  in  simple 
hydatogenic*  deposits.  Such  are  minerals  like  fmorite, 

*  Water-formed. 


108  GEOLOGY  APPLIED  TO  MINING. 

tourmaline  and  topaz,  containing  the  volatile  elements 
boron  and  fluorine.  Garnet  is  also  a  rather  characteristic 
mineral  of  these  deposits.  According  to  W.  Lindgren,* 
a  characteristic  feature  is  the  association  of  oxides  of 
iron  with  sulphides,  f 

Example:  The  ores  at  the  old  Tungsten  mine,  Trumbull, 
Connecticut,  have  been  formed  at  the  contact  of  an  igneous 
rock  (now  metamorphosed  to  hornblende  gneiss)  and  a 
limestone  into  which  the  igneous  rock  was  intrusive.  The 
ore  occurs  on  the  contact  of  these  two  rocks,  in  beds  from 
3  to  5  feet  thick.  It  consists  of  quartz  containing  iron 
pyrites,  epidote,  calcite,  mica,  and  the  wolfram  minerals 
(scheelite  and  wolframite)  for  which  the  mine  has  been 
worked.  Zoisite,  garnet,  scapolite,  hornblende,  and  rnar- 
casite,  are  also  found.  This  ore-bearing  contact  zone  was 
due  to  the  action  of  solutions  at  the  contact  of  the  intruded 
igneous  rock,  these  solutions  being  both  heated  and  under 
pressure.! 

Associated  veins  are  of  pegmatite  or  vein  quartz,  and 
contain,  besides  feldspar,  muscovite  and  other  common 
minerals,  topaz  in  large  masses,  fluorite,  etc. 

What   are  some  of  the  special  results  of  vapors  and  gases  in 
the  residual   material  under  discussion,  as  regards  deep- 
seated  deposits? 
The  deposits  formed  by  the  material  expelled  from  cooling 

igneous  rock  probably  vary  according  to  the  nature  and  the 

* 'Character  and  Genesis  of  Certain  Contact-Deposits.'  Transactions 
American  Institute  Mining  Engineers,  Vol.  XXXI,  p.  227. 

t  As  contemporarily  formed  minerals. 

t  W.  H.  Hobbs,  22d  Annual  Report  United  States  Geological  Survey,  Part 
II,  p.  13. 


IGNEOUS     ROCKS.  109 

relative  abundance  of  the  gases.  Pegmatites  usually  show, 
from  the  presence  of  certain  minerals  containing  well- 
known  gases  in  their  composition,  the  presence  and  agency 
of  those  gases  in  their  formation.  Tin-veins  and  veins  con- 
taining other  valuable  minerals,  metallic  and  earthy,  are 
probably  often  formed  largely  by  the  action  of  abundant 
gases,  escaping  from  cooling  granular  rocks,  deep  below 
the  surface. 

What  characteristic  evidence  as  to  their  origin  do  veins  of  this 

class  offer? 

The  characteristic  sign  of  the  origin  of  these  veins  is  the 
presence  of  minerals  which  do  not  easily  form  under  simple 
aqueous  conditions,  but  are  commonly  produced  by  the 
action  of  vapors.  Among  these  minerals  are  tin-stone 
(cassiterite,  oxide  of  tin)  itself,  and  others  like  tourmaline, 
topaz,  etc.  Many  apatite  (phosphate  of  lime,  with  fluorine 
or  chlorine)  deposits  also  probably  belong  to  this  class,  as 
the  presence  of  the  volatile  elements  (chlorine  and  fluorine) 
in  the  mineral  itself  indicates.  One  of  the  commonest 
minerals  found  in  association  with  the  apatite,  that  is, 
scapolite,  bears  the  same  evidence,  for  the  scapolites  are 
minerals  whose  composition  is  practically  the  same  as  the 
feldspars,  save  for  the  presence  of  chlorine,  indicating  the 
agency  of  this  gas  at  the  time  of  formation. 

Are  ores  deposited  also  by  vapors  and  gases  escaping  from 

volcanic  rocks  at  or  near  the  surface? 

At  the  surface,  orifices  emitting  vapors  and  gases  from 
cooling  volcanic  rocks  are  termed  fumaroles.  From  these 


110  GEOLOGY  APPLIED  TO  MINING. 

gases  valuable  earthy  and  metallic  minerals  may  be  depos- 
ited.    This  fumarolic  activity  may  persist  for  ages. 

Alunite  or  natural  alum  (sulphate  of  aluminum  and 
potash),  for  example,  is  formed  in  commercially  valuable 
quantities  by  the  action  of  sulphurous  gases  on  igneous  rocks 
containing  potash  and  aluminum.  Sulphur,  deposited  in 
this  way,  is  actively  exploited  in  Italy  and  elsewhere.  On 
the  walls  of  fissures  in  lava,  where  steam  and  other  gases 
escape,  various  metallic  minerals  have  been  found  which 
are  due  to  this  action,  such  as  specular  iron  (hematite, 
oxide  of  iron),  cinnabar  (sulphide  of  mercury),  realgar 
(sulphide  of  arsenic),  etc.  Other  ores  very  likely  are 
sometimes  formed  in  this  way. 

Example:  The  Bassick  mine  is  in  Custer  county,  Colo- 
rado. The  rock  of  the  region  is  gneiss,  but  an  explosive 
volcano  has  broken  through,  producing  a  pipe  which  is  now 
filled  with  rounded  boulders,  chiefly  of  volcanic  rock.  In 
places  these  boulders  are  coated  with  rich  metallic  minerals. 
The  first  coat  consists  of  lead,  antimony,  and  zinc  sulphides ; 
an  intermediate  coat  is  of  zinc  sulphide,  rich  in  silver  and 
gold;  other  coats  are  of  chalcopyrite  (sulphide  of  copper 
and  iron),  and  pyrite  (sulphide  of  iron).  Tellurides  of  the 
precious  metals  also  occur.  These  ores  are  very  generally 
considered  by  geologists  to  have  been  brought  up  in  the 
form  of  vapor  during  the  fumarolic  activity  of  the  volcano. 


IGNEOUS     ROCKS.  Ill 

SPECIAL   RELATIONS   BETWEEN  CERTAIN 
IGNEOUS  ROCKS  AND  ORE-DEPOSITS. 

ADVANTAGES  OF  DIFFERENT  FORMS  OF  IGNEOUS  ROCKS. 

What  especial  phases  do  the  processes  attendant  upon  cooling 
show  in  fundamental  igneous  rocks f 

Fundamental  rocks  are  generally  of  coarse  grain,  showing 
slow  consolidation.  Since  they  have  cooled  in  many  cases 
at  great  depth,  gases  and  vapors  attending  this  process  may 
have  formed  characteristic  ore-deposits.  Tin- veins,  for 
example,  are,  as  noted,  generally  confined  to  such  rocks, 
of  granitic  composition. 

What  advantages  or  disadvantages  do  extrusive  rocks  possess 
for  ore-concentration? 

Extrusive  rocks  and  hot  springs  are  allied  occurrences; 
yet,  unless  the  flows  of  these  rocks  are  very  thick,  they  are 
too  near  the  surface  for  hot-spring  action  to  exercise  its 
best  effort  in  producing  mineralization ;  for  the  heat  of  the 
rocks  disappears  with  comparative  rapidity,  and  with  it 
the  great  concentrating  power  of  the  waters.  Where  the 
flows  are  very  thick,  however,  the  central  portions  remain 
warm  for  a  very  long  time,  and  the  hot-spring  action  is 
prolonged  and  becomes  more  productive  of  results. 

Fumarolic  activity,  properly  speaking,  and  fumarolic 
deposits  are  confined  to  extrusive  rocks;  while  contact 
metamorphic  deposits  are  lacking. 


112  GEOLOGY   APPLIED   TO   MINING. 

What  advantages  have  intrusive  rocks  over  others  in  bringing 

about  ore-deposition? 

Intrusive  rocks  are  the  most  favorable  for  promoting  the 
formation  of  ores.  Like  all  igneous  rocks,  they  contain 
disseminated  metals.  On  account  of  their  being  distant 
from  the  surface,  they  cool  with  comparative  slowness, 
especially  if  they  are  in  bodies  of  considerable  size;  and 
thus  the  conditions  for  ore  concentration  are  prolonged. 

Intrusions  come  in  contact  with  other  igneous  rocks  and 
with  stratified  rocks.  While  the  igneous  rocks  are  better 
fitted  than  the  sedimentaries  for  instigating  the  processes 
of  ore-deposition  and  for  furnishing  the  disseminated  metals 
to  the  mineralizing  waters,  yet  the  latter  are  more  suitable 
for  precipitating  the  dissolved  metals.  This  is  due  to  the 
easy  dissolution  and  replacement  of  the  limestones,  to  the 
tiny  pores  of  the  sandstones,  which  permits  interstitial 
deposition,  and  to  the  organic  matter  of  the  shales,  which 
often  acts  as  a  direct  precipitant.  Hence,  where  the  two 
classes  of  igneous  and  sedimentary  rocks  are  intimately 
associated,  the  most  favorable  conditions  are  realized. 

ADVANTAGES  OF  DIFFERENT  KINDS  OF  IGNEOUS  ROCKS. 

Preferences  of  Certain  Igneous  Rocks  for  Certain  Ores, 
Displayed  During  the  Cooling  Processes. 

Are  the  dark  basic  rocks  more  closely  associated  with  ore- 
deposits  than  the  light  silicious  ores? 
It  has  been  found  by  chemists  who  have  investigated  the 

metallic  contents  of  fresh  igneous  rocks  that  the  metals 


IGNEOUS     ROCKS.  113 

were  mostly  present  in  the  dark  "ferro-magnesian"  min- 
erals— hornblende,  pyroxene,  black  mica,  olivine,  etc.  This 
being  the  case,  we  might  expect  ore-deposits  to  be  more 
definitely  associated  with  the  dark-colored  basic  rocks  than 
with  the  silicious  ones.  But  this  is  only  partly  the  case;  the 
preference  seems  to  depend  chiefly  on  the  kind  of  metal. 

Are  some  metals  preferentially  associated  with  light-colored 

and  silicious  rocks? 

Tin  is  usually  found  in,  or  in  relation  with,  granite;  and  a 
general  close  connection  with  silicious  rocks  seems  the  case 
with  tungsten  and  molybdenum. 

Example:  In  the  Malay  Peninsula  tin  deposits  are  found, 
mainly  on  the  western  slope  of  the  mountain  range  that 
forms  the  backbone  of  the  peninsula.  This  range  is  com- 
posed largely  of  granitic  rocks,  with  some  limestone  and 
sandstone.  The  ore  is  cassiterite,  associated  with  tourma- 
line, hornblende,  tungsten  minerals,  magnetite,  muscovite, 
topaz,  fluorite,  sapphire,  etc.  Veins  are  found  generally 
in  the  granite,  less  frequently  in  the  other  rocks.* 

Are  some  metals  associated  by  preference  with  dark-colored 

and  basic  rocks? 

Chromium  ore-deposits  (chromite),  for  example,  are 
hardly  found  save  in  very  basic  rocks — peridotites. 

When  these  peridotitic  rocks  decompose,  they  become 
serpentine,  which  accounts  for  the  chrome  deposits  fre- 
quently occurring  in  serpentine  rock. 

Iron  (generally  magnetic,  often  containing  titanium)  also 


*R.  A.  F.  Penrose  Pacific  Coast  Miner  Vol.  VII,  p.  340. 


114          GEOLOGY  APPLIED  TO  MINING. 

forms  ore-deposits  in  many  basic  rocks.  As  these  rocks 
are,  by  their  very  definition,  richer  in  iron  than  the  silicious 
ones,  iron  deposits  in  general  may  be  allowed  to  exhibit  a 
certain  preference  for  them. 

Copper  usually  prefers  basic  rocks;  on  the  other  hand, 
there  are  many  instances  of  rich  copper  deposits  in  silicious 
rocks. 

Example:  In  Cuba,  serpentine  is  abundant  among  the 
most  ancient  rocks.  The  serpentine  is  of  igneous  origin, 
being  derived  from  the  alteration  of  dark,  basic,  igneous 
rocks  (such  as  peridotite).  This  rock  is  and  has  been 
considered  the  most  productive  of  metals- among  the  for- 
mations of  the  island.  It  contains  large  deposits  of 
copper,  ores  of  iron  and  chromium,  and  gold.* 

Platinum  was  formerly  only  found  in  placers.  In  Russia, 
however,  some  years  ago,  the  metal  was  found  as  an  original 
constituent  in  peridotitic  rocks,  and  late  inquiry  in  America 
has  fixed  it  as  being  in  a  number  of  such  rocks.  Prof. 
Kemp  has  reported  platinum  in  peridotite  from  the  Tula- 
meen  region,  British  Columbia.  It  was  also  reported  from 
"fine-grained  dark  basaltic  rock"  in  British  Columbia  in 
1895  by  Mr.  Carmichael,t  assayer  for  British  Columbia. 
It  seems,  therefore,  to  be  chiefly  confined  to  the  basic 
rocks,  and  it  is,  indeed,  an  intimate  associate  of  chromite. 
Yet  it  has  been  found  to  occur,  though  less  abundantly, 
even  in  so  silicious  a  rock  as  syenite. 

*  Fernandez  de  Castro,  Hayes,  Vaughan,  and  Spencer.      'Geological  Recon- 
naisance  of  Cuba,'  1901. 

f  Engineering  and  Mining  Journal,  Feb.  12,  1902,  p.  249. 


IGNEOUS     ROCKS.  115 

Most  of  the  other  minerals  have,  as  far  as  known,  slight 
or  no  preference  for  certain  igneous  rocks. 

Preferences  of  Certain  Igneous  Rocks  for  Certain  Ores, 

Displayed  by  Selective  Precipitation  of  Metals 

from  Solution. 

May  ores  show  a  preference  for  one  igneous  rock  over  another, 
on  account  of  the  different  effect  of  different  rocks  in  pre- 
cipitating ores  from  solution? 

When  ore-bearing  solutions  traverse  a  variety  of  igneous 
rocks,  there  will  be  certain  chemical  reactions  between  the 
so  utions  and  the  walls  of  the  fracture  which  has  afforded 
them  a  channel.  Where  the  rock  is  porous  and  permeable, 
so  that  the  solutions  spread  out  and  traverse  it  thoroughly 
and  slowly,  there  the  opportunity  for  such  reactions  is  very 
great.  In  igneous  rocks  of  different  mineralogical  and 
chemical  composition,  the  -same  solutions  will  react  in 
different  ways,  and  the  substances  precipitated  from  solu- 
tion as  a  result  of  these  reactions  will  be  apt  to  differ,  both 
in  quantity  and  quality.  The  result  will  be  variable,  and 
will  depend  as  much  on  the  specific  character  of  the 
solutions  (which  vary  greatly)  as  on  the  character  of  the 
rock. 

Thus  it  may  happen  that  an  ore-bearing  solution  will 
form  a  rich  deposit  in  one  igneous  rock  and  in  another, 
along  the  same  fracture,  very  little.  Moreover,  on  account 
of  the  different  character  of  solutions,  a  certain  rock  may 
in  one  case  be  selected  by  preference  for  ore-deposition,  and 
in  another  case  may  be  specially  avoided. 


116 


GEOLOGY  APPLIED  TO  MINING. 


Example:  At  Butte,  Montana,  two  granites  of  different 
ages  occur,  one  of  which  is  ten  per  cent,  more  silicious  than 
the  other.  The  less  silicious  granite  contains  a  consid- 
erable amount  of  hornblende  and  biotite,  while  the  other 
contains  very  little.  An  important  class  of  copper  ores 
here  have  formed  by  replacement  of  the  igneous  rocks. 
The  fractures,  along  which  circulated  the  solutions  that 
deposited  the  ore,  cut  both  rocks.  In  the  less  silicious 
granite,  the  veins  are  commonly  rich  in  copper;  in  the  more 
silicious  granite  they  are  almost  equally  wide  and  strong, 
but  are  lean,  and  composed  chiefly  of  quartz,  with  com- 


-il  Granite  «-*»"'!  Aplite  K$'&'f\  Quartz 

Fig.  8.    Ideal  plan  of  conditions  in  a  copper  vein  at  Butte,  Montana,  passing 
from  less  silicious  granite  into  silicious  granite.     After  W.  H.  Weed. 

paratively  little  pyrite  and  copper.  Microscopic  study  of 
the  rocks  show  that  in  the  process  of  replacement  the  horn- 
blende was  the  first  mineral  to  be  altered  to  ore,  indicating 
that  the  nature  of  the  solutions  were  such  as  to  react  most 
readily  with  this  mineral.  It  is  believed  that  the  presence 
of  the  hornblende  and  other  dark  minerals  in  the  less 
silicious  granite,  and  their  absence  in  the  more  silicious 
rock,  determined  the  preferential  precipitation  of  the  ores 
in  the  former.*  (Fig.  8.) 


*  W.  H.  Weed,   Transactions  American  Institute  Mining  Engineers,  Vol. 
XXXI,  p.  643. 


IGNEOUS     ROCKS.  117 


ORE-BODIES  IN  THE  ROLE  OF  INTRUSIVE  ROCKS. 

Are  metallic  minerals  ever  thrust  up  in  a  molten  condition  in 
the  form  of  dikes,  requiring  no  further  concentration  to  form 
ores? 

It  has  been  explained  how  certain  metallic  minerals  may 
become  segregated  in  molten  masses  so  as  to  form  ore- 
deposits.  If  these  metallic  segregations,  instead  of  remain- 
ing where  they  originate,  are  disturbed  by  some  movement, 
and  forced  up  into  the  rock  above  while  still  wholly  or 
partly  fluid,  it  is  conceivable  that  we  should  have  dikes  of 
ore.  The  occurrence  of  iron  ore  (magnetite)  in  this  form 
has  actually  been  reported. 

Example:  On  Calamity  brook,  near  lake  Sanford,  in  the 
Adirondack  Mountains,  are  dikes  of  titaniferous  magnetite 
in  anorthosite  (a  granular  rock  composed  almost  wholly  of 
labradorite,  a  species  of  feldspar).  The  ore  in  the  hand 
specimen  appears  to  be  an  exceedingly  ferriferous  gabbro, 
and  it  contains  inclusions  of  anorthosite,  through  which 
run  little  dikes  of  pyroxene,  garnet,  and  ore  that  end  in 
streaks  of  pyrites.  The  anorthosite  inclusions  are  believed 
to  be  masses  of  the  country  rock  which  were  torn  off  during 
the  intrusion  of  the  ore  and  about  and  through  wHch 
gaseous  action  developed  the  little  dikes,  and  streaks  of 
pyrites.  Thin  sections  of  the  ore-dikes,  studied  under  the 
microscope,  show  coarsely  crystalline  aggregates  of  ilmenite 
or  titaniferous  magnetite,  pyroxene,  and  a  little  biotite. 
Regarded  as  ores,  they  vary  in  richness,  being  sometimes 
nearly  pure  magnetite,  and  again  more  than  half  silicates.* 

*J.  F.  Kemp,  19th  Annual  Report  United  States  Geological  Purvey,  Part 
III,  p.  412, 


118 


GEOLOGY   APPLIED   TO    MINING. 


IGNEOUS    ROCKS    INTRUSIVE    SUBSEQUENT   TO 
ORE-DEPOSITION. 

May  not  intrusive  igneous  rocks  sometimes  form  later  than  an 
ore-body,  and  so,  though  closely  associated  with  it,  yet  have 
no  part  in  its  formation? 

Just  as  faults  may  be  earlier  than  ore-deposition  in  a 
certain  case,  and  may  furnish  the  channels  along  which  the 
ore  is  concentrated,  or  may  be  later  than  ore-deposition, 
may  cut  and  displace  the  ore-body,  and,  far  from  being  a 


Fig.  9.  Iron  ore-bodies  (hematite  and  magnetite),  Lola  mine,  Santiago  prov- 
ince, Cuba.     Black  portion  is  ore,  surrounded  by  porphyry. 
After  Hayes,  Vaughan,  and  Spencer. 

help  in  the  ore-concentration,  may  be  only  a  hindrance 
and  vexation  to  the  miner — just  so  an  intrusive  igneous 
rock  may  be  earlier  than  ore-deposition  and  be  largely 
responsible  for  it,  or  may  be  later  and  cut  it  up  and  separate 
it. 

Where  two  igneous  rocks  are  intruded  at  various  times 


IGNEOUS     ROCKS.  119 

in  the  same  place,   the  ore-deposit  resulting  from  the 
influence  of  the  first  intrusion  may  be  broken  by  the  second. 

Example:  The  hematite  and  magnetite  iron  ores  of 
Santiago  province,  Cuba,*  have,  since  their  formation,  been 
cut,  floated  up,  and  surrounded  by  intrusive  masses  of 
porphyry,  so  as  to  entirely  alter  their  form  (Fig.  9). 

*  Hayes,  Vaughan,  and  Spencer.     'Geological  Reconnaissance  of  Cuba,' 
1091,  p.  81. 


CHAPTER  IV. 

THE  STUDY  OF  DYNAMIC  AND   STRUCTURAL 
GEOLOGY  AS  APPLIED  TO  MINING. 


PART  I. 

GENERAL  CONCEPTIONS  AND  MAPPING. 
DEFINITIONS. 

What  is  dynamic  geology  and  structural  geology? 

Dynamic  geology  is  a  study  of  the  physical  forces  which 
produce  changes  in  the  earth's  crust.  These  forces  (due 
to  the  contraction  of  the  earth  from  cooling,  to  migrations 
of  molten  rock  beneath  the  solid  crust,  or  to  the  unequal 
weight  of  different  features  of  the  surface,  bringing  about 
unstable  equilibrium)  produce  bending  and  breaking  in  the 
rocks.  Such  disturbance  is  mainly  noticeable  in  the 
stratified  rocks,  the  beds  of  which  are  forced  out  of  their 
original  horizontal  position  into  all  manner  of  folds;  or  they 
are  even  broken,  with  one  part  thrust  past  the  other  along 
the  line  of  fracture.  This  last  is  called  a  fault.  The  study 
of  the  arrangement  or  structure  of  these  bent  and  broken 
rocks  is  called  structural  geology. 


DYNAMIC    AND   STRUCTURAL   GEOLOGY.  121 

FOLDS  AND  FAULTS. 

What  is  the  meaning  of  the  term  dip? 

The  inclination  of  a  bed  (or  other  geological  feature 
having  a  plane  direction)  is  the  dip,  which  is  measured  in 
degrees  from  the. horizontal. 

Should  we  use  the  word  hade  instead  of  dip,  in  speaking  of 

veins? 

The  inclination  of  veins,  dikes,  faults,  etc.,  is  also  called 
hade,  and  is  measured  in  degrees  from  the  vertical.  There 
is,  however,  no  need  to  have  two  opposing  terms  for  any 
one  thing,  and  it  is  better  to  apply  the  term  dip  to  the 
inclination  of  the  veins,  faults,  etc.,  as  well  as  strata,  and  to 
measure  it  in  the  same  way. 

What  are  the  principal  kinds  of  folds? 

Folds  are  chiefly  divided  into  two  kinds,  according  to 
whether  they  are  open  above  or  below.  These  are  called 
respectively  synclines  and  anticlines. 

A  line  drawn  from  the  apex  of  a  fold  (the  highest  point 
of  an  anticline  or  the  lowest  point  of  a  syncline),  midway 
between  the.  two  sides  or  limbs  of  a  fold  lies  in  the  axis 
(Fig.  10). 

If  the  axis  is  vertical,  the  two  sides  (or  limbs)  have  the 
same  dip ;  if  the  axis  is  inclined  the  dips  of  the  limbs  will  be 
unequal,  unless  they  are  parallel,  as  when  the  folding  19 
intense  and  they  have  been  jammed  together,  forming  a 
compressed  or  close  fold,  (Fig.  11), 


122          GEOLOGY  APPLIED  TO  MINING. 

The  opposite  of  a  close  fold  is  an  open  fold  (Fig.  10). 

What  are  overthrown  folds? 

In  open  folds  the  limbs  normally  dip  in  opposite  direc- 
tions; yet  the  folds  may  be  such  that  the  limbs  incline  in 


Fig.  10.   Folding  of  limestones  and  shales  on  Kuskokwim  river,  Alaska.     After 

J.  E.  Spurr,*  a.=anticline;  b.=anticline  overthrown  at  the  apex; 

c.=faulted  anticline;  dd.=synclines. 


Fig.  11.  Close  folding  in  limy  shales  on  Yukon  river,  Alaska,  below  Mission 
creek.     After  J.  E.  Spurr.f 


the  same  direction,  though  not  necessarily  at  the  same 
angle.  This  constitutes  an  overthrown  fold  (Fig.  12).  In 
extreme  cases  the  axis  may  assume  a  horizontal  position. 


*  20th  Annual  Report  United  States  Geological  Survey.  Part  VII,  p.  127. 
1 18th  Annual  Report  United  States  Geological  Survey,  Part  III,  p.  177. 


DYNAMIC    AND   STRUCTURAL   GEOLOGY. 


123 


What  is  a  monocline? 

Where  strata  suddenly  change  from  a  horizontal  to  an 
inclined  position  and  then  become  horizontal  again,  a  fold 
with  only  one  limb — a  monocline — is  formed  (Fig.  13). 


Fig.  12.   Overthrown  folds ;  aa.  anticlines;  ss.  synclines. 


Fig.  13.  Monoclinal  fold. 

What  is  a  normal  fault  and  what  is  a  reversed  fault? 

Most  fault  planes  have  an  inclination  or  dip  between  the 
horizontal  and  vertical.  When  the  rocks  on  the  upper  side 
have  moved  down,  relative  to  the  rock  on  the  under  side, 
the  fault  is  called  normal.  If  the  reverse  movement  has 
taken  place,  the  fault  is  called  a  reversed  or  thrust  fault. 


124 


GEOLOGY   APPLIED   TO    MINING. 


The  majority  of  faults  are  normal;  but  reversed  faults  are 
frequent  (Figs.  14  and  15). 

What  are  compensating  faults? 

Where  a  stratum  or  vein  is  faulted  in  many  places  it 
sometimes  happens  that  one  fault  will  displace  the  bed,  and 


14.  Faults  in  strata,  near  Forty  Mile,  Yukon  river,  Alaska;  nn.=normal 
faults;  r.=reversed  fault.     After  J.  E.  Spurr.* 


Quartz 


Fig.  15.  Reversed  fault,  longitudinal  section,  Empire  mine,  Grass  Valley,  Cali- 
fornia.    After  W.  Lindgren.f 

another  will  bring  it  back  to  its  original  position.  The  two 
faults  are  thus  compensating.  In  other  words,  the  block 
comprised  between  the  two  faults  has  been  moved  out  of 
line,  leaving  the  rest  in  place. 


*  18th  Annual  Report  United  States  Geological  Survey,  Part  III,  p.  177, 
1 17th  Annual  Report  United  States  Geological  Survey,  P*rt  II,  p.  253. 


DYNAMIC    AND   STRUCTURAL    GEOLOGY. 

Example:  The  accompanying  figure  represents  compen- 
sating faults  in  the  Omaha  mine,  Grass  Valley,  California. 
These  faults  displace  a  vein  about  one  foot  wide,  consisting 
of  quartz  containing  galena  and  iron  pyrite,  and  other 
sulphides,  with  some  free  gold  (Fig.  16).* 

What  connection  have  faults  with  folds? 
A  monocline  may  easily  pass  into  a  fault.     Faults  along 


N 


.  16.  Longitudinal  section,  showing  fault,  Omaha  mine,  Graha  mine,  Grass 
Valley,  California.     After  W.  Lindgren. 


the  axes  of  folds  are  also  common,  for  along  the  axes  rocks 
are  weakened  by  bending  and  therefore  liable  to  break. 
The  directions  of  faults  are  likely  to  conincide  with  those  of 
folds  in  the  same  region,  for  they  both  may  originate  as  a 
result  of  the  same  kind  of  pressure. 


*  Waldernar    Lindgren.      17th  Annual   Report    United    States    Geological 
Survey,  Part  II,  p.  243. 


126  GEOLOGY  APPLIED  TO  MINING. 

EFFECTS  OF  EROSION  ON  FOLDED  AND  FAULTED 
ROCKS. 

What  is  erosion? 

Erosion  may  be  defined  as  the  process  of  wearing  away. 
As  applied  to  geology,  it  signifies  the  wearing  away  of  the 
rocks  at  the  surface,  chiefly  by  the  action  of  streams. 

Are  folds  and  faults  visible  as  such  at  the  surface? 

Deformation  (as  folding  and  faulting  taken  together  may 
be  called)  would  affect  the  earth's  surface  precisely  as  it 
does  the  rocks,  were  it  not  for  the  counteracting  effects  of 
erosion.  Erosion  is  a  slow  process,  but  it  is  continuous. 
Folding  and  faulting  is  also  a  slow  process — rarely  spas- 
modic. It  goes  on  beneath  our  feet  today  so  gently  that 
we  do  not  notice  it  except  where  there  is  a  slip  in  the  gentle 
mechanism  and  an  earthquake  results.  Sometimes  defor- 
mation is  more  rapid  in  moulding  the  earth's  surface  than 
is  erosion;  sometimes  erosion  is  the  more  active.  But 
after  deformation  has  stopped,  owing  to  the  easing  of  the 
deforming  pressure,  erosion  still  keeps  on,  so  in  the  end  it 
mostly  has  its  own  way  and  shapes  the  minor  features  of 
the  earth  to  suit  itself.  Therefore,  folds  and  faults  are 
sometimes,  but  not  usually,  directly  expressed  as  such  at 
the  surface. 

What  are  the  effects  of  erosion  and  deformation  in  producing 

topographic  features? 

Erosion  and  deformation  are  usually  in  opposition;  the 
latter  lifts  up  mountains  while  the  former  is  engaged  in 


DYNAMIC    AND   STRUCTURAL   GEOLOGY. 


137 


wearing  them  down.  Both  forces,  even  if  each  were  left  to 
itself,  tend  to  produce  irregularities — ridges  and  furrows — 
in  the  earth's  surface.  Simple  deformation  makes  upfolds 
(anticlines)  which  are  mountains,  and  downfolds  (synclines) 
which  are  valleys.  In  the  work  of  erosion  rivers  cut  deep 
trenches  which  are  valleys,  and  the  high  parts  left  be- 
tween are  the  mountains.  The  first  named  are  mountains 


Fig.  17.  Graded  anticlinal  range  of  deformation.     A  generalized  transverse  sec- 
tion of  the  Uinta  range,  Utah.     After  C.  A.  White.* 

and  valleys  of  deformation;  the  second,  mountains  and 
valleys  of  erosion. 

Example:  The  Uinta  range,  in  Utah,  as  shown  in  the 
accompanying  figure  (Fig.  17),  is  an  anticlinal  range,  the 
upfold  in  the  strata  corresponding  with  the  topographic 
dome.  The  great  thickness  of  stratified  rocks  between  the 
dotted  line  on  the  figure  and  the  present  mountain  tops  (a 
thickness  greater  than  the  height  of  the  mountains  above 
the  plains)  has  been  stripped  off  by  erosion.  Still,  if  the 

*  9th  Annual  Report  United  States  Geological  Survey,  p.  694. 


128          GEOLOGY  APPLIED  TO  MINING. 

present  relief  of  the  range  is  directly  due  to  the  upfolding  of 
the  crust,  as  geologists  have  held,  this  is  a  range  of  de- 
formation. 


Are  mountains  of  erosion  upfolds  of  the  crust? 

Mountains  of  erosion  are  not  necessarily  upfolds.  Upfolds 
tend  to  weaken  the  rocks  so  that  they  are  more  easily 
washed  away,  leaving  valleys,  with  synclinal  mountains 
between.  Mountains  may  be  composed  of  upfolds  and 
downfolds  together;  and  they  may  trend  diagonally  or  at 
right  angles  to  the  trend  of  the  folds. 

In  general,  what  relation  has  topography  to  folds? 

As  the  result  of  these  inharmonious  processes,  we  may 
expect  to  find  the  relief  or  topography  bearing  any  con- 
ceivable relation  to  the  structure.  In  a  hilly  or  mount- 
ainous region  the  structure  is  sometimes  suggested  by  the 
topography,  but  more  frequently  the  topography  only 
obscures  its  elucidation.  It  even  frequently  happens  that 
a  highly  folded  region  may  have  become  by  long  erosion 
topographically  a  plain. 

What  is  the  relation  between  topography  and  faults? 

As  with  folds,  so  with  faults.  Faults  break  the  earth's 
surface  and  the  moving  of  one  rock  past  the  other  produces 
a  cliff  or  scarp  (simple  fault-scarp).  Only  recent  faults 
show  this  (Fig.  18). 

The  erosion  which  attacks  a  faulted  surface  may  do  one 
of  several  things,  dependent  on  the  different  nature  of  the 
rocks  on  either  side  of  the  fault  brought  together  by  the 


DYNAMIC    AND    STRUCTURAL    GEOLOGY. 


129 


movement.  There  may  result  a  scarp  (erosion  fault- 
scarp),  a  gully  or  valley  along  the  fault,  or  the  fault  may 
not  influence  at  all  the  outlines  of  the  topography.  An 
erosion  fault-scarp  is  produced  when  the  rock  on  one  side 


m 


Fig.  18.  Simple  fault-scarp  at  the  Palisades,  Yukon  river,  Alaska; 
scarp;  b.=fault.     After  J.  E.  Spurr.* 


Fig.  19.  Reversed  erosion  fault-scarp.     Section  in  the  Lower  Austrian  Alps. 
After  Bittner. 

of  the  fault  is  softer,  and  so  more  easily  worn  away,  than  on 
the  other.  If  the  rock  on  the  upthrown  side  of  the  fault 
is  harder  than  that  on  the  downthrown  side,  the  scarp  will 

*  18th  Annual  Report  United  States  Geological  Survey,  Part  III,  p.  199. 


130  GEOLOGY  APPLIED  TO  MINING. 

face  the  downthrown  side,  that  is,  it  will  simulate  a  simple 
fault-scarp.  This  may  be  called  a  normal  erosion  fault- 
scarp.  But  if  the  rock  on  the  downthrown  side  is  harder, 
it  will  eventually  become  higher  by  the  wearing  away  of 
the  upthrown  side,  and  the  scarp  will  face  this  latter  side. 
This  is  a  reversed  erosion  fault-scarp  (Fig.  19). 

Since  therefore  we  cannot  tell  beforehand  what  relation  the  rock 
structure  will  have  to  the  topography,  how  are  we  to  work 
out  the  structure  problems? 
Structure   can   be   satisfactorily   worked   out   only   by 

reasoning  from  the  attitude  and  position  of  the  rocks  as 

they  appear  at  the  surface  or  outcrop. 

THE  SURFACE  MANTLE  OF  DEBRIS. 
Do  rocks  outcrop  all  over  the  surface? 

When  we  start  out  to  study  the  geology  of  a  district,  we 
find  here  one  rock  and  there  another;  here  a  bed  with  a 
certain  inclination,  there  another  bed  inclining  with  a 
different  angle  in  another  direction;  then  soil  and  forests 
without  outcrops,  valley  bottoms  free  from  hard  rock,  etc. 
The  underground  rocks  do  not  outcrop  continuously  save 
in  high  mountainous  regions. 

Why  do  not  rocks  outcrop  continuously? 

Exposed  rocks  break  up  at  the  surface,  under  the  influ- 
ence of  heat  and  cold,  frost  and  thaw,  rain  and  wind,  the 
roots  of  trees  and  plants,  and  the  decomposing  acids,  chiefly 
derived  from  vegetation,  which  soak  down  into  the  rocks 
and  attack  them.  The  result  is  that  such  rocks  crumble 


DYNAMIC    AND    STRUCTURAL    GEOLOGY.  131 

into  sand  and  clay.  Vegetation  takes  root,  flourishes  and 
dies,  and  new  generations  of  plants  arise;  thus  a  top  loam 
is  formed,  by  the  accumulation  of  vegetable  remains.  This 
loose  decomposed  material,  or  soil,  is  often  found  in  place, 
directly  over  the  solid  rock  whence  it  is  derived.  But,  on 
account  of  its  looseness,  it  usually  moves  downhill,  into 
the  valleys,  and  out  towards  the  sea,  in  a  steady  but  very 
leisurely  journey.  Thus  steep  mountain  tops  become 
stripped  and  expose  only  fresh  rock,  while  their  lower  slopes 
are  covered  thickly  with  coarse  fragments,  and  the  valleys 
below  are  deeply  filled  with  soil  (wash  or  drift).  Farther 
down  the  valleys,  towards  the  sea,  th's  wash  is  apt  to  cover 
larger  and  larger  areas  of  solid  rock  (bed-rock)  until  at  last 
it  rests  in  the  sea  and  there  builds  up  a  new  series  of  sedi- 
ments. 

Besides  the  rocks  that  go  to  pieces  slowly  and  thoroughly, 
a  great  deal  is  broken  up  more  suddenly  and  violently  by 
rapid  mountain  streams. 

Are  glaciers  active  in  making  soil  and  gravel? 

Thousands  of  years  ago,  there  existed  a  great  continental 
glacier,  (like  that  which  now  covers  much  of  Greenland 
so  deeply  that  we  do  not  know  where  the  land  leaves  off 
beneath  it  and  the  sea  commences)  over  most  of  British 
North  America  east  of  the  Rockies,  and  reached  down  into 
the  United  States.  Its  southern  limit  extended  on  the 
east  into  New  Jersey,  while  on  the  extreme  west  it  hard- 
ly got  below  the  present  northern  boundary  of  the  United 
States.  In  the  mountains  of  some  of  our  Western  States 
such  as  Washington  and  Oregon,  we  still  have  local  gla- 


132 


GEOLOGY    APPLIED    TO    MINING. 


DYNAMIC    AND    STRUCTURAL    GEOLOGY.  133 

ciers,  occupying  valleys,  broad  mountain  sides  or  coastal 
slopes.  Alaska  contains  many  such  glaciers,  some  of 
which  cover  thousands  of  square  miles. 

Glaciers  are  powerful  rock  crushers  and  erosive  agents, 
and  in  their  slow  imperceptible  forward  flow  they  leave  the 
material,  which  they  have  crushed  and  mingled,  either 
beneath  them  or  along  their  margins.  Therefore  the  region 
of  the  old  continental  glaciers  (the  glaciated  area)  is  gener- 
ally thickly  covered  by  broken  and  mixed  rock  and  soil 
(glacial  drift)  from  which  the  bed-rock  peeps  out  only  in 
places  (Fig.  20). 

Does  this  covering  of  soil  and  gravel  make  the  unravelling  of 

the  structure  difficult? 

On  the  high  mountains,  where  the  rock  is  all  exposed,  it  is 
possible  for  an  observer  of  ordinary  keenness  to  perceive  the 
structure,  unless  it  is  complicated;  but  in  a  country  where 
outcrops  are  not  abundant  it  is  difficult  to  read  even  simple 
structure. 

THE  SYSTEMATIC  WORKING  OUT  OF  GEOLOGIC 
STRUCTURE. 

STRIKE  AND  DIP. 

How  shall  one  start  to  work  out  the  structure  of  folded  and 

faulted  rocks? 

To  work  out  the  structure  of  a  region,  one  must  first  learn 
to  take  the  strike  and  dip  of  stratified  rocks,  for  these  rocks 
furnish  the  best  key  to  the  disturbances  which  the  crust  has 
undergone  since  their  deposition.  We  know  that  they  were 


134 


GEOLOGY  APPLIED  TO  MINING. 


laid  down  horizontally :  hence,  when  we  find  them  tilted  a«t  a 
certain  angle,  we  know  that  the  crust  at  this  point  has  been 
deformed  to  this  extent. 

How  does  one  record  strike  and  dip? 

The  dip  has  already  been  defiried  as  the  inclination  of  a 
bed,  measured  in  degrees,  from  the  horizontal.  The  strike 
is  the  direction  of  the  outcropping  edge  of  an  inclined  bed, 
on  a  horizontal  surface  (such  as  it  would  be  on  a  flat  plain) 


Fig.  21.  The  directions  of  strike  and  dip. 


and  is  generally  recorded  in  degrees  from  the  north  or  from 
the  south  (Fig.  21).  The  writer  prefers  referring  all  read- 
ings so  far  as  convenient,  to  the  north  point, — thus,  N.  10° 
E.,  N.  90°  E.,  N.  75°  W.,  etc.  The  direction  of  the  dip  is 
invariably  at  right  angles  to  the  strike,  but  the  inclination 
may  be  to  one  side  or  the  other  of  the  line  of  str'ke.  Hence, 
in  recording  the  dip,  it  is  only  necessary  to  note  the  general 
direction  (the  exact  direction  being  known  from  the  strike) 
and  the  angle  of  inclination.  Thus,  strike  N.  60°  E.,  dip 
25°  N.  W.;  or  strike  N.  35°  W.,  dip  6°  S.  W. 


DYNAMIC    AND    STRUCTURAL    GEOLOGY.  135 

How  accurately  should  strike  and  dip  be  read? 

It  is  generally  useless  to  read  the  strike  and  dip  closer 
than  a  degree;  for  the  attitude  of  a  bed  generally  varies 
constantly,  though  often  slightly,  so  that  greater  accuracy 
in  one  place  will  not  help  in  the  broader  problems. 

How  should  the  strike  be  read? 

For  this  work  a  hand-compass,  having  a  clinometer 
(arrangement  for  reading  the  dip),  is  sufficient.  To  read 
the  strike,  get  the  line  of  sight  of  the  compass  (the  line 
between  the  sights,  or  the  straight  edge  of  a  square  com- 
pass) parallel  to  a  line  mads  by  a  horizontal  plane  cutting 
the  surface  of  a  bed,  and  read  the  angle  between  this  and 
the  north  point  of  the  compass  needle.  Thus,  if  this  line  is 
30°  to  the  right  of  the  north  point  of  the  needle,  (the  obser- 
ver facing  in  a  northerly  direction),  its  direction  is  N.  30°  E. 
magnetic. 

The  reading  may  be  corrected  subsequently  so  as  to  read 
to  the  true  north,  by  applying  the  known  magnetic  varia- 
tion. The  angle  of  this  variation  is  to  be  added  to  the  angle 
read,  or  subtracted  from  it,  according  to  whether  the  varia- 
tion in  the  region  under  examination  is  to  the  east  or  to  the 
west  of  the  true  north.  For  example,  in  Maine  at  a  place 
where  the  variation  (declination)  is  18°  W.  (that  is,  where 
the  magnetic  needle  points  18°  W.  of  true  north)  a  magnetic 
strike  reading  N.  30°  E.  would  be  corrected,  by  subtracting 
the  variation,  to  N.  12°  E.  true;  in  Oregon,  at  a  place  where 
the  variation  is  20°  E.,  the  same  magnetic  reading  (N.  30°  E. 
would  be  corrected,  by  adding  the  variation,  to  N.  50°  E. 
true. 


136  GEOLOGY  APPLIED  TO  MINING. 

How  should  the  dip  be  read? 

To  read  the  dip  there  is,  in  the  clinometer  compass,  a 
little  weight  which  hangs  down  and  so  is  vertical.  One  side 
of  the  square  compass  being  held  parallel  to  the  dip  of  the 
rock,  the  number  of  degrees  of  this  dip  from  the  horizontal 
(or  from  the  vertical,  if  one  pleases)  is  registered  on  a  scale 
across  which  the  weight  swings.  There  are  other  clino- 
meter arrangements,  but  for  ordinary  geological  work  this 
simple  one  is  as  good  as  any.  A  graduated  scale  of  degrees 
pasted  on  the  cover  of  a  notebook,  with  a  small  coin  at  the 
end  of  a  thread,  as  weight,  will  also  answer  the  purpose,  the 
straight  edge  of  the  book  being  held  parallel  with  the  dip  in 
measuring. 

How  can  one  best  find  the  horizontal  line? 

In  measuring  strike  and  dip,  it  is  best  to  judge  with  the 
eye  the  general  horizontal  direction  and  greatest  inclination 
of  an  outcrop,  and  to  hold  the  compass  in  the  hand,  away 
from  the  outcrop,  as  near  these  average  directions  as 
possible.  The  strike  and  dip  usually  vary  much  locally. 
Sometimes  it  is  easiest  to  scratch  a  horizontal  line  on  the 
exposed  face  of  a  bed,  to  get  the  true  strike.  One  may 
remember  that  the  dip  is  always  the  greatest  inclination  of 
abed. 

RECORDING  OBSERVATIONS  ON  MAPS. 

What  is  necessary  for  the  continuance  of  the  work? 

The  next  necessary  thing  is  to  have  some  sort  of  map.  If 
there  is  none  on  the  proper  scale,  a  small-scaled  map  may 
be  enlarged,  and  corrected  as  one  works.  Where  there 


DYNAMIC   AND   STRUCTURAL   GEOLOGY.  137 

is  none  at  all,  a  sketch  map  will  have  to  serve.  For  such  a 
sketch  map,  directions  are  easily  found  with  a  compass. 
Distances  are  got  by  pacing,  with  or  without  a  pedometer; 
by  an  odometer  attached  to  a  carriage,  or  a  cyclometer  on  a 
bicycle.  Elevations  can  be  taken  with  an  aneroid  barom- 
eter. 

For  accurate  work,  an  accurate  map  is  necessary.  Such 
a  map  is  most  easily  made  with  a  plane-table,  a  method  by 
which  the  surveying  and  the  plotting  go  on  simultaneously. 
The  principal  points  are  determined  by  triangulation,  the 
elevations  by  means  of  vertical  angles  read  with  a  transit, 
(or  alidade  of  a  plane-table)  from  the  chief  stations,  and 
the  minor  points  sketched  and  re-sketched  from  the  different 
stations  until  they  are  approximately  correct.  For  still 
more  accurate  work  the  position  and  elevation  of  nearly  all 
the  points  are  determined  by  stadia  work.  In  these  last  two 
ways  are  made  the  beautiful  contoured  maps  of  the  United 
States  Geological  Survey.  Levelling  may  also  be  used  for 
determining  elevations.  A  contoured  map,  or  at  least  one 
where  the  chief  elevations  are  definitely  recorded,  is  essen- 
tial to  any  but  the  rudest  of  geological  work,  for  it  enables 
the  student  afterward  to  read  and  reconstruct  the  topog- 
raphy, without  which  the  geology  as  exhibited  on  a  plane 
map, — a  projection  of  the  real  surface  on  a  horizontal 
plane — can  hardly  be  understood. 

What  does  the  student  record  on  this  map? 

Upon  this  base-map  the  student  should  record  every 
outcrop  which  he  judges  necessary.  In  a  complicated 
country  or  where  outcrops  are  few,  often  every  rock 


138  GEOLOGY  APPLIED  TO  MINING. 

exposed  is  necessary.  But  where  there  are  many  outcrops 
of  the  same  strike  and  dip  and  of  the  same  kind  of  rock,  or 
where  the  structure  is  simple,  many  may  be  omitted. 

The  outcrops  which  are  recorded  may  generally  be 
located  on  the  map  (especially  a  detailed  map)  by  locating 
the  topography  of  the  place  in  question — i.  e.,  if  the  outcrop 
occurs  on  top  of  a  hill,  and  the  top  of  that  hill  is  shown  on 
the  map,  one  can  place  the  outcrop  as  closely  as  necessary. 
Where  there  are  few  landmarks,  it  is  often  necessary  to 
locate  outcrops  instrumentally,  by  means  of  intersection 
from  two  known  points,  or  by  a  direction  and  measured 
distance  from  some  one  known  point,  the  result  being 
plotted  on  the  map  according  to  the  scale  used. 

How  are  strike  and  dip  recorded? 

For  recording  the  strike  and  dip,  the  following  sign  is 
commonly  used,  the  long  line  being  the  strike,  and  the 
arrow,  with  the  angle  written  close  to  it,  recording  the 
direction  and  inclination  of  the  dip  (Fig.  22). 


Fig.  22. 

How  should  the  different  rocks  be  plotted  f 

The  kind  of  rock  may  be  written  on  the  map,  but  it  is 
better  to  use  an  arbitrary  sign  for  each  rock,  or,  better  still, 
a  color.  A  box  of  colored  pencils  may  be  used  for  this,  and 
each  color  may  be  taken  to  represent  one  of  the  .'mportant 
rocks  of  the  district  in  question.  For  example,  blue  can  be 
used  for  limestone,  brown  for  quartzite,  red  for  granite, 
and  so  on. 


DYNAMIC    AND    STRUCTURAL    GEOLOGY.  139 

When  all  the  data  are  thus  plotted,  does  it  help  our  compre- 
hension of  the  structure? 

When  all  available  outcrops  have  been  recorded,  the 
general  distribution  of  each  color,  representing  its  particular 
formation,  will  be  shown.  In  this  way  it  often  can  be  pre- 
dicted, frequently  with  great  accuracy,  what  rocks  underlie 
the  coverings  of  soil  and  glacial  drift  or  valley  wash,  where 
there  are  no  outcrops.  By  extending  the  line  of  strike  in 
different  outcrops  of  the  same  formation  till  they  come 
together,  the  extension  of  beds  under  covering  materials 
can  be  made  out  with  especial  certainty.  If  there  is  great 
and  uniform  disconnection  along  a  certain  line,  between  the 
strikes  of  such  outcrops  thus  extended,  then  the  geologist 
knows  that  this  line  is  a  fault-line,  though  it  may  not  be 
visible  to  the  eye  (on  account  of  covering  material) ;  and 
even  the  amount  of  discordance,  or  displacement  of  the 
fault,  can  often  be  closely  calculated. 

MIGRATION  OF  OUTCROPS. 

Do  the  lines  of  outcrop  of  veins,  faults,  etc.,  on  the  surface} 
always  give  an  accurate  idea  of  their  direction? 

One  must  cultivate  some  geometrical  perception  to  grasp 
the  true  attitude  of  beds,  dikes,  faults,  etc.,  from  the 
puzzling  lines  of  outcrops  afforded  by  the  ordinary  topo- 
graphic surface,  especially  where  this  is  irregular.  The 
surface  is  a  very  uneven  plane,  which  cuts  these  beds,  dikes 
and  faults  at  all  angles,  and  since  they  themselves  lie  at  all 
angles,  the  intersections  may  be  infinitely  varied.  A  bed 
or  fault  having  a  straight  strike  may  have  an  outcrop 


140  GEOLOGY   APPLIED   TO    MINING. 

which  will  describe  many  kinds  of  curves  when  represented 
on  the  geologic  map.  The  problem  is :  Given  a  plane  cutting 
an  uneven  surface,  where  will  be  the  intersection? — the 
plane  being  the  bed,  dike  or  fault,  and  the  uneven  surface 
the  surface  of  the  ground.  Since  the  latter,  is  always 
changing,  the  problem  does  also. 

What  is  the  explanation  of  this  outcrop  migration? 

On  a  perfectly  plane  portion  of  the  earth's  surface, 
another  plane,  such  as  a  sedimentary  bed,  dike,  vein  or 
fault,  will  outcrop  as  a  staight  line,  whatever  its  dip.  The 
only  plane  land  surface  which  we  find  in  nature  continuing 
for  a  long  distance  is  a  horizontal  plain.  Here  then  a  bed 
will  outcrop  in  a  straight  line,  following  the  direction  of 
the  strike.  As  soon  as  irregularities  come  in,  the  outcrop 
abandons  the  straight  line  and  wanders  in  irregular  curves 
and  angles.  This  is  so  because  the  further  up  an  inclined 
bed  is  cut  the  further  the  outcrop  moves  horizontally  in  a  direc- 
tion opposite  from  the  dip;  the  further  down  it  is  cut  the 
more  the  outcrop  advances  in  the  direction  of  the  dip.  The 
amount  of  the  outcrop  migration  depends  on  the  dip  of  the 
bed.  In  a  vertical  bed  it  is  zero ;  in  a  horizontal  bed  infinity. 
It  is  necessary  to  be  familiar  with  these  laws,  for  often 
when  an  outcrop  occurs  it  is  important  to  know,  both  in 
geologic  mapping  and  in  practical  exploring  and  mining 
operations,  what  is  its  course  over  a  topographically 
irregular  country,  where  slide-rock  (talus),  gravel  wash, 
glacial  drift,  or  vegetation  renders  continuous  actual  obser- 
vation impossible. 


DYNAMIC   AND   STRUCTURAL   GEOLOGY.  141 

How  can  one  estimate  the  amount  of  outcrop  migration  where 
continuous  observation  is  impossible? 

By  trigonometry,  it  is  easy  to  find  how  much  the  outcrop 
of  a  bed  of  given  dip  will  advance  with  the  dip  out  of  the 
line  of  strike,  or  retreat  away  from  the  dip  out  of  it,  with  a 
given  heightening  and  lowering.  The  change  in  height 
may  be  taken  as  the  peipendicular  side  of  a  right  triangle, 
and  the  dip  as  the  angle  opposite  the  perpendicular.  Then 
the  base  is  the  horizontal  migration  of  an  outcrop  (or  the 
actual  migration  as  projected  on  to  a  horizontal  map),  and 
the  hypothenuse  is  the  actual  migration,  as  measured 
roughly  on  the  surface,  in  an  airline  between  the  two  exten- 
sions of  two  outcrops,  and  at  right  angles  to  the  strike.  The 
first  measurement  (the  horizontal  migration),  is  exclu- 
sively used  in  mapping;  but  a  cross-section  constructed 
from  the  map  shows  the  actual  migration  graphically.  In 
practical  work  an  estimation  of  both  the  horizontal  and 
actual  migration  will  be  often  of  value. 

The  formulas  are  as  follows: 

Horizontal  migration  =  change  in  height  multipled  by 
the  cotangent  of  the  d'p. 

Actual  migration  =  change  in  height  divided  by  the  sine 
of  the  dip. 
.    Taking  the  change  in  height  as  1 : 

Horizontal  migration  =  cotangent  of  dip. 

Actual  migration  =  the  reciprocal  of  the  sine  of  the  dip. 

Following  is  a  table  for  the  most  important  dips: 

The  figures  are  calculated  for  a  change  in  height  of  1  unit. 
The  concrete  example  of  100  feet  has  been  taken. 


142 


GEOLOGY  APPLIED  TO  MINING. 


Dip  of  bed, 
dike,  vein, 
etc. 

Change  of 
height  in 
topographic 
surface 

Horizontal 
migration 
of 
outcrop. 

Actual 
migration 
of 
outcrop. 

0° 

5° 

100ft. 
100ft. 

infinity 
1143ft. 

infinity 
1147  ft. 

10° 

100ft. 

567  ft. 

576  ft. 

15° 

100ft. 

373  ft. 

386  ft. 

20° 

100ft. 

275  ft. 

292  ft. 

25° 

100ft. 

215  ft. 

237  ft. 

30° 

100ft. 

173  ft. 

200  ft. 

35° 

100ft. 

143  ft. 

174  ft. 

40° 

100ft. 

119  ft. 

155  ft. 

45° 

100ft. 

100  ft. 

141  ft. 

50° 

100ft. 

84ft. 

130  ft. 

55° 

100ft. 

70ft. 

122  ft. 

60° 

100ft. 

58ft. 

115  ft. 

65° 

100ft. 

47ft. 

110ft. 

70° 

100ft. 

36ft. 

106  ft. 

75° 

100ft. 

27ft. 

103  ft. 

80° 

100ft. 

18ft. 

101  ft. 

85° 

100ft. 

9ft 

100  ft. 

90° 

100ft. 

Oft. 

100  ft. 

CONSTRUCTION  OF  GEOLOGIC  SECTIONS. 

After  plotting  observations  on  maps,  what  is  the  next  step? 

The  next  step  toward  the  comprehension  of  the  structure 
is  the  construction  of  vertical  sections.  Cross-sections  (at 
right  angles  to  the  line  of  strike)  are  the  most  serviceable. 


DYNAMIC   AND   STRUCTURAL   GEOLOGY.  143 

Where  should  the  cross-sections  be  placed? 

These  should  be  placed,  first,  where  the  surface  outcrops 
give  the  most  thorough  data,  and,  second,  neither  in  the 
most  simple  nor  in  the  most  complicated  places  (for  the 
first  sections,  at  least). 

How  is  the  base  for  cross-sections  constructed? 

The  base  for  such  sections  is  taken  from  the  topographic 
maps.  From  a  straight  line,  corresponding  in  length  to  the 
length  of  the  line  taken  on  the  map,  perpendiculars  are 
drawn  at  the  points  where  there  are  on  the  map  data  as  to 
relative  elevation.  These  relative  elevations  are  then 
measured  off  on  the  perpendiculars,  from  the  base  line. 
The  elevation  of  the  base  line  may  be  stated  as  related  to 
sea-level,  where  this  is  known,  or  to  some  other  datum 
plane;  or  the  line  may  be  given  an  assumed  elevation. 

Should  the  vertical  scale  be  different  from  the  horizontal? 

Frequently  the  scale  used  for  plotting  these  elevations  is 
greater  (twice,  three 'times,  ten  times  as  great)  than  that  of 
the  base  line — i.e.,  the  vertical  scale  is  greater  than  the 
horizontal.  This  gives,  in  the  sections,  exaggerated  topo- 
graphy and  exaggerated  dips  to  the  strata  represented. 
But  in  most  cases  it  is  better  to  have  the  vertical  scale 
the  same  as  the  horizontal,  especially  in  large  scale  work 
and  in  mining  work;  this  gives  a  more  accurate,  even  if 
less  accentuated,  representation  of  the  structure. 

How  is  the  outline  of  the  topography  obtained? 

By  drawing  a  line  connecting  the  points  thus  marked  out 


144 


GEOLOGY  APPLIED  TO  MINING. 


Fig.  23.  Method  of  cons- 
truction of  a  topographic 
base  for  geologic  cross- sec- 
tions. Scale  1  inch=1000 
feet. 


on   the   verticals,    a  section  of  the 
topography  results  (Fig.  23). 

How  are  the  geologic  data  put  on  this 
topographic  section? 

On  this  section  the  geological  data 
found  on  this  same  line,  on  the  geo- 
logic map,  are  plotted,  the  stratified 
rocks  showing  their  dip.  In  places 
where  there  are  no  data  on  this  line, 
outcrops  not  too  far  away  may  be 
represented  by  prolonging  their  line 
of  strike  till  they  intersect  the  sec- 
tion line.  As  in  the  map,  different 
rocks  may  be  represented  by  differ- 
ent colors. 

The  inspection,  on  this  section, 
of  the  dips  of  a  bed  which  outcrops 
at  various  places  generally  allows  a 
correct  reading  of  the  structure,  and 
an  easy  deduction  of  the  attitude  and 
location  of  the  bed  beneath  other 
rocks,  away  from  its  outcrops. 
Thus  the  outcrops  may  be  connected 
and  the  complete  structure  repre- 
sented. Mines,  wells,  bore  holes, 
etc.,  are  all  of  the  highest  value  for 
supplementing  and  fixing  the  elu- 
cidation of  structure. 


DYNAMIC   AND  STRUCTURAL  GEOLOGY.  145 

Cross-sections  should  be  made  at  intervals,  at  a  con- 
venient distance  apart.  Often  one  well-established  cross- 
section  helps  in  the  working  out  of  the  neighboring  ones. 

Are  longitudinal  sections  ever  advisable? 

Longitudinal  sections,  parallel  with  the  strike  of  the 
stratified  rocks  and  at  right  angles  to  the  cross-section,  are 
sometimes  valuable.  Like  the  cross-sections,  they  are 
taken  vertically  and  are  constructed  in  the  same  manner. 
Cross-sections  and  longitudinal  sections  cross  each  other, 
and  at  the  lines  of  intersection  should  be  identical,  so,  when 
made  independently,  they  are  a  valuable  check  on  one 
another.  When  the  data  for  one  section  are  incomplete, 
an  intersecting  section  may  often  supply  it  with  data 
which  will  enable  its  being  worked  out.  Both  cross  and 
longitudinal  sections,  when  worked  out,  help  to  correct  the 
surface  geological  map,  and  to  establish  more  accurately 
the  boundaries  of  the  different  formations,  where  the'se  are 
concealed. 

ECONOMIC  RESULTS  OF  MAPPING  AND  CROSS-SECTIONING. 

What  is  the  economic  application  of  this  mapping  and  cross- 
sectioning? 

In  this  way  it  can  be  ascertained  what  is  the  course  of  an 
economically  valuable  bed,  such  as  one  of  coal,  iron,  salt, 
borax,  oil  or  water,  bedded  veins  of  various  metallic 
minerals,  etc.  The  approximate  position  of  such  beds  can 
be  established  beneath  coverings  of  drift,  and  the  proper 
places  for  sinking  shafts  to  find  these  outcrops  can  be 


146  GEOLOGY  APPLIED  TO  MINING. 

determined.  The  sinking  of  shafts  or  the  driving  of  tunnels 
in  barren  rock,  lying  next  to  the  bed  sought  after,  can  be 
planned,  and  the  distance  that  these  drivings  must  be 
pushed  to  reach  the  bed  may  be  determined  beforehand. 
The  same  system,  more  carefully  carried  out,  enables  the 
tracing  of  an  ordinary  lode  or  vein  outcrop  under  drift 
coverings;  it  makes  the  searching  for  the  underground 
continuation  of  the  surface  outcrop  of  a  vein,  by  means  of 
new  shafts  and  tunnels,  in  many  cases,  a  matter  of  close 
calculation  instead  of  guesswork. 

What  amount  of  geologic  knowledge  is  necessary  in  order  to  be 
able  to  make  such  valuable  studies? 

To  make  such  geologic  studies  it  is  necessary  to  be  able 
to  recognize  the  different  formations  in  the  field — to  dis- 
tinguish sandstone,  shale,  conglomerate,  limestone  and 
quartzite  from  one  another,  and  to  have  as  much  of  an 
idea  of  igneous  rocks  as  is  given  in  the  preceding  chapter. 
Close  determination  of  the  rocks  is  usually  not  necessary, 
except  for  detailed  work.  The  recognition  of  the  relation  of 
the  ores  to  a  certain  rock  in  the  district  in  question,  be  that 
rock  what  it  may,  and  enough  science  to  follow  that  important 
rock  both  at  the  surface  and  under  it,  is  the  essential  thing.  It 
does  not  concern  the  miner,  in  many  cases,  what  the  age  of 
the  rock  is.  One  may  observe  that  in  a  certain  silver-lead 
district  the  ore-bodies  are  generally  in  or  near  a  certain 
shale-bed — the  problem  is  to  follow  that  bed  everywhere. 
In  another  district,  one  may  remark  that  the  ores  occur 
chiefly  near  faults;  then  the  problem  is  to  search  for  the 


DYNAMIC   AND   STRUCTURAL   GEOLOGY.  147 

faults  of  the  district,  to  study  which  have  been  the  most 
favored  by  ore-deposition,  and  to  inquire  into  their  hori- 
zontal and  vertical  extent. 

MAPPING  AND  SECTIONING  OF  IGNEOUS  ROCKS. 

Can  one  reason  out  the  underground  continuation  and  position 
of  igneous  rocks  in  the  same  way  as  sedimentary  rocks? 
One  must  remember,  in  reasoning  out  structure,  that 
sedimentary  rocks  conform  to  one  another — that  is,  their 
bedding  planes  are  parallel,  for  they  were  laid  down  one  on 
top  of  the  other,  on  the  sea-bottom.  But  igneous  rocks  are 
not  necessarily  parallel  to  one  another;  neither  do  their 
boundaries,  unlike  those  of  the  sedimentary  rocks,  have 
any  constant  direction.  Thus  it  is  difficult  to  reason  out 
accurately  the  outlines  of  an  igneous  body  beneath  the 
surface;  though  the  general  ideas  sketched  in  the  last 
chapter  will  usually  enable  an  approximation.  One  can 
decide  whether  the  rock  is  a  surface  flow,  the  outcrop  of  a 
dike  or  sill  or  irregular  mass,  or  is  a  fundamental  body,  and 
so  can  draw  his  conclusions  as  to  the  underground  exten- 
sion. Generally  the  direction  and  dip  of  dikes  can  be 
obtained  from  their  outcrops;  and  a  study  of  the  fault,  fold 
and  joint  systems  in  the  rock  frequently  throws  some  light 
on  the  dike  system  also,  for  all  are  apt  to  be  related. 


PART  II. 

ROCK    DEFORMATION    AND    DISLOCATION,    AND 
THEIR   CONNECTION  WITH  MINERAL  VEINS. 


MEASUREMENT  OF  FOLDS  AND  FAULTS. 

Do  rock  folds  have  only  two  dimensions? 

Folds  and  faults  must  be  thought  of,  not  only  as  they  are 
represented  on  cross-sections,  in  two  dimensions,  but  in  all 
their  three  dimensions.  Think  of  a  sheet  of  paper  folded 
and  crumpled — the  folds  will  not  always  be  regular  along 
the  strike,  but  there  will  be  uneven  ridges  and  hollows. 
In  geology  a  ridge,  from  which  the  rocks  dip  away  on  all 
sides,  so  that  every  section  is  anticlinal,  is  called  a  dome; 
a  hollow,  of  which  every  section  is  synclinal,  is  a  basin.  It 
is  possible,  however,  that  an  irregular  fold  may  be  anti- 
clinal in  one  section ;  and  in  another,  at  right  angles  to  the 
first,  synclinal. 

When  can  the  displacement  of  a  fault  be  estimated? 

Where  there  is  a  number  of  different  rocks,  such  as 
distinct  beds,  which  the  fault  separates,  it  is  only  necessary 
to  match  the  beds  on  one  side  of  the  fault  with  the  same 
beds  on  the  other  side,  to  know  approximately  how  much 
they  have  been  separated.  The  contacts  of  igneous  rocks, 


DYNAMIC     AND     STRUCTURAL     GEOLOGY  149 

or  dikes,  or  faulted  veins  or  ore-bodies,  or  even  faulted 
faults  (the  fault  having  cut  through  and  displaced  a  pre- 
existing older  fault,  as  sometimes  happens),  may  also  be 
matched  on  the  two  sides  of  a  fracture  to  measure  its 
movement. 

•• 

7s  the  separation  of  the  parts  of  a  faulted  sedimentary  bed 
always  an  accurate  measurement  of  the  amount  of  displace- 
ment? 

A  fault  may  lie  in  any  plane  (for  example,  it  may  be 
parallel  to  a  sedimentary  bed  or  perpendicular  to  it) ,  and  on 
this  plane  the  direction  of  movement  may  be  represented 
by  any  conceivable  line.  It  may  even  be  parallel  to  the 
plane  of  the  sedimentary  beds  cut  by  the  fault,  in  which 
case  the  beds  will  not  be  separated,  no  matter  how  great 
the  movement;  but  a  dike  cutting  these  beds  at  right 
angles  will  be  displaced  by  the  whole  movement  of  the 
fault  (Fig.  28).  It  is  only  when  the  plane  of  the  sedimentary 
beds  is  perpendicular  to  the  direction  of  faulting  that  the 
separation  of  the  parts  of  a  given  bed  is  an  accurate 
measurement  of  the  movement. 

Do  vertical  cross-sections  show  accurately  the  displacement  of 
a  fault? 

One  is  apt  to  consider  faults  simply  as  dislocations  of 
sedimentary  beds,  and  to  assume  that  the  amount  of 
movement  which  appears  on  vertical  sections  is  the  whole 
displacement.  The  amount  of  displacement  thus  shown  is 
easily  found  graphically,  and  is  valuable  as  showing  the 


150  GEOLOGY  APPLIED  TO  MINING. 

existence  of  a  fault,  and  the  break  it  makes  in  the  sediment- 
ary beds;  but  it  does  not  necessarily  convey  an  accurate 
idea  of  the  whole  displacement. 


Is  a  more  accurate  measure  of  fault-displacement  necessary  to 
mining  work? 

Suppose  that  a  spherical  or  lenticular  or  irregular  ore- 
body  is  cut  in  the  middle  by  a  fault,  and  one  half  of  the 
ore  having  been  worked  out  up  to  the  fault  plane,  it  is 
the  question  to  find  where  the  other  half  has  gone.  In  this 
case  the  separation  of  the  strata  (if  the  ore  is  in  sedimentary 
rocks)  as  seen  in  a  vertical  section,  gives  absolutely  no  clue 
as  to  either  the  direction  or  the  amount  of  'displacement. 
In  mining  work,  therefore,  we  must  study  more  closely. 

Can  we  measure  fault-displacement  in  homogeneous  rock- 
masses? 

In  homogeneous  rock-masses  the  amount  of  movement  in 
faults  cannot  be  ascertained  or  even  approximately  esti- 
mated. The  existence  of  a  movement  can  be  determined 
by  the  record  left  on  the  slipping  surface  or  surfaces,  in  the 
shape  of  ground  up  rock,  (fault-breccia),  of  polished  and 
scratched  (striated)  rock  surfaces  (slickensides)  etc.  But 
the  amount  of  friction  shown  by  grinding  and  rubbing  is  not 
necessarily  proportionate  to  the  amount  of  movement,  for 
some  faults  with  slight  displacement  have  thick  crushed 
zones,  while  others  of  far  greater  movement  show  the 
effects  of  friction  to  a  slight  degree  only. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY  151 

Can  we  accurately  measure  fault-displacement  in  a  heteroge- 
neous rock  mass? 
In  a  rock  mass  composed  of  different  kinds  of  rocks,  we 

may  measure  with  a  greater  or  less  degree  of  accuracy  the 

amount  of  movement. 

What  are  the  chief  aids  in  the  work  of  measuring  faults? 

In  mining  geology  it  has  been  found  that  the  more  valu- 
able aids  are  (besides  sed'mentary  beds):  igneous  bodies, 
such  as  dikes;  veins;  bodies  of  ore;  pre-existing  faults; 
scratches  (striae)  on  the  fault  plane,  showing  the  direction 
of  movement;  and  the  composition  of  the  fault-breccia, 
which  may  show,  in  some  degree,  the  direction  and  the 
amount  of  movement. 

How  do  these  things  afford  the  necessary  data? 

The  first  four  guides  to  displacement  above  mentioned 
are  applicable  because  any  continuous  geologic  feature, 
when  broken  and  displaced,  may  be  matched  in  imagina- 
tion by  the  observer. 

As  regards  the  striae,  one  rock  moving  past  another  along 
a  fracture  will  mark  the  other  rock  with  grooves  parallel  to 
the  direction  of  movement.  A  given  fault  may  have  a 
strike  N.  45°  E.  and  a  dip  of  30°  to  the  north-west.  On 
this  fault  plane  we  may  find  that  the  striae  are  nearly  hori- 
zontal. We  then  know  that  along  this  fracture  the  faulted 
portions  moved  horizontally  past  each  other.  But  we  do 
not  yet  know  in  which  horizontal  direction  a  given  side 
moved.  Did  the  rock  on  the  southeast  side  of  the  fault 
move  to  the  northeast  or  to  the  southwest?  This  can 


152  GEOLOGY  APPLIED  TO  MINING. 

sometimes  be  told  from  a  careful  inspection  of  the  striae. 
Scratches  that  are  narrow  and  deep  at  one  end  and  become 
broad  and  shallow  at  the  other  are  usually  caused  by 
movements  toward  the  shallow  side  on  the  part  of  the 
rock  which  did  the  scratching;  and,  conversely,  indicate 
movement  toward  the  sharp  end  for  the  side  which  bears 
the  scratches. 

Regarding  fault-breccia,  we  may  take  as  example  a 
faulted  ore-body  which  leaves  in  the  breccia  a  "trail"  of 
ore,  indicating  the  direction  of  movement.  Concerning  the 
breccia  as  a  test  for  the  amount  of  movement,  the  following 
may  be  said :  If  we  find  fragments  of  a  certain  rock,  such  as 
a  granite  or  a  sandstone,  in  the  fault-breccia  at  a  point 
where  the  wall  rocks  are  both  of  different  nature  from 
these  fragments,  then  the  movement  must  have  been  at 
least  as  great  as  the  distance  from  these  fragments  to  the 
nearest  place  where  granite  or  sandstone  forms  one  of  the 
walls  of  the  fault. 

Can  faults  be  directly  measured  from  the  data  in  question? 

Sometimes  the  fault-movement  may  be  directly  measured 
from  the  aids  above  mentioned ;  but  more  often  it  must  be 
calculated  from  at  least  two  of  them.  The  direct  measure- 
ment is  possible,  when  the  two  parts  of  a  separated  ore- 
body  have  been  found  on  the  two  sides  of  a  fault,  or  where 
the  intersection  of  a  given  dike  with  a  given  stratum  has,  in 
the  same  way,  been  found  on  both  sides,  or  where  it  is 
otherwise  possible  to  identify  any  given  point  on  the  two 
sides.  With  zones  of  homogeneous  rock,  such  as  beds, 
dikes  and  veins,  the  identification  of  any  point  is  difficult 


DYNAMIC     AND     STRUCTURAL     GEOLOGY  153 

— hence  the  unreliability  of  these  bodies  alone  as  registrars 
of  the  true  movement. 

What  are  the  functions  of  a  fault  movement,  and  how  can  they 
be  calculated? 

The  following  functions  of  a  fault  movement  are  im- 
portant: 

Dislocation  and  displacement  are  general  terms,  appli- 
cable to  any  part  or  the  whole  of  a  fault  movement.  Each 
of  the  functions  defined  below,  and  to  which  specific  names 
are  given,  may  be-  called  simply  a  dislocation  or  displace- 
ment. 

Total  displacement  is  the  distance  which  two  points, 
originally  adjacent,  are  separated  by  the  fault  movement; 
the  line  connecting  these  two  points  lies  in  the  faultplane 
in  all  straight  faults.  It  is  occasionally  possible  to  deter- 
mine the  total  displacement  directly  by  such  criteria  as  are 
mentioned  above;  but  ordinarily  it  can  only  be  calculated 
or  approximately  estimated  from  some  of  its  more  easily 
measured  functions. 

Example:  The  total  displacement  of  a  fault  can  best  be 
represented  by  a  diagram.  Fig.  24  shows  a  block  of  the 
earth's  crust,  which  is  represented  for  the  purpose  of 
illustration  as  being  transparent.  In  the  figure  a  portion 
of  a  given  sedimentary  bed  is  represented,  traversed  by  a 
mineral-bearing  vein  (or  it  may  be  a  dike  of  igneous  rock). 
This  bed  and  included  vein  are  cut  by  a  given  fault  plane, 
and  the  movement  on  the  fault  plane  is  such  that  the  vein, 
at  its  intersection  with  the  bed,  is  separated  in  the  direction 
and  by  the  distance  a  b.  This  distance  is  the  real  (maxi- 


154 


GEOLOGY  APPLIED  TO  MINING. 


mum)  fault-movement,  or  total  displacement.  On  the 
earth's  surface  c  is  the  outcrop  of  the  fault  plane,  d  of  the 
sedimentary  bed,  and  e  of  the  vein. 

The  lateral  separation  is  the  perpendicular  or  shortest 
distance  between  the  two  parts  of  any  continuous  zonal 
body  (such  as  a  sedimentary  bed),  which  has  been  separated 
by  a  fault,  the  distance  being  measured  along  the  fault 


Fig.  24.  Stereogram  illustrating  tb.e  total  displacement  of  a  fault. 

plane.  The  lateral  separation  may  be  measured  in  a 
vertical,  horizontal,  or  oblique  line,  according  to  the 
attitude  of  the  bodies  between  which  it  is  measured,  and 
in  any  fault  it  may  vary  from  zero  to  the  total  displace- 
ment. The  total  displacement  may  often  be  calculated 
from  the  lateral  separation,  since  the  latter  is  always  the 
side  of  a  right  triangle  of  which  the  former  is  the  hypotenuse. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY  155 

Example:  The  lateral  separation  of  a  fault  is  shown  in 
Fig.  25,  where  it  is  represented  by  the  dotted  line  be,  while 
the  total  displacement  is  represented  by  the  line  ab. 

The  perpendicular  separation  is  the  perpendicular  dis- 
tance between  the  corresponding  planes  in  the  two  parts  of  a 
single  body  available  as  criterion  (such  as  a  sedimentary 
bed),  when  this  body  has  been  separated  by  a  fault,  the 
planes  on  each  side  of  the  fault  being  projected  for  the 
purpose  of  measuring,  if  necessary. 

Example:  To  illustrate  the  term  perpendicular  separa- 
tion let  us  take  Fig.  25.  This  is  an  ideal  representation  of  a 


Fig.  25.  Stereogram  to  illustrate  various  functions  of  a  fault,    ab  is  total  dis- 
placement; be  is  lateral  separation;  db  is  perpendicular  separation. 

portion  of  a  sedimentary  bed  which  has  been  faulted  along 
the  short  straight  edges  of  the  pieces,  so  that  the  pieces 
come  to  occupy  the  relative  position  shown.  Then  the 
perpendicular  distance  between  the  two  separated  planes, 
represented  by  the  dotted  line  db,  is  the  perpendicular 
separation.  If  the  fault  was  in  the  opposite  direction,  so 
that  the  broken  pieces  were  separated  by  a  gap  instead  of 
overlapping,  then  one  of  the  planes  wouM  have  to  be 
projected  in  order  to  measure  the  perpendicular  separation. 


156  GEOLOGY  APPLIED  TO  MINING. 

The  perpendicular  separation  thus  has  a  certain  relation 
to  the  lateral  separation;  for  it  constitutes  a  side  of  a  right 
triangle,  the  hypotenuse  of  which  is  the  lateral  separation, 
except  in  the  possible  case  where  the  perpendicular  and 
lateral  separation  coincide. 

This  mathematical  relation  makes  it  often  possible  to 
estimate  the  lateral  separation  from  the  perpendicular 
separation,  and  from  the  latter  the  total  displacement. 

Example:  To  illustrate  the  calculation  of  one  of  these 
measurements  from  another,  let  us  look  again  at  Fig.  25, 
where  ab,  the  actual  fault  movement  (being  the  distance 
by  which  the  two  portions  of  the  intersection  of  the  dike 
with  the  sedimentary  bed  are  separated)  is  the  total  dis- 
placement, be  (drawn  along  the  fault  plane,  perpendicular 
to  the  edge  of  the  faulted  bed,  and  hence  the  shortest  line 
that  can  be  drawn  along  the  fault  plane  between  the 
broken  edges)  the  lateral  separation,  and  db,  the  perpen- 
dicular distance  between  the  planes  of  the  separated 
portions,  the  perpendicular  separation.  Then  cdb  is  a 
right  triangle,  as  is  bca. 

Suppose  a  case  that  may  often  happen,  that  most  of  the 
figure  represented  is  concealed,  as  shown  by  the  shading  in 
Fig.  26,  only  the  light  portion  (which  may  represent  an 
outcrop  or  a  mining  shaft  or  tunnel)  being  displayed.  We 
may  in  any  case  measure  the  perpendicular  separation. 
Then,  taking  the  angle  of  the  fault  plane  with  the  faulted 
stratum,  we  may  calculate  the  lateral  separation;  for  this 
angle  deducted  from  90°  gives  the  angle  dbc  (Fig.  25). 
Then  the  perperjdicular  separation  db  divided  by  the  cosine 
of  dbc  equals  be,  the  lateral  separation  Suppose,  again, 
the  fault  plane,  as  shown  in  Fig,  26,  is  scratched  (striated) 
or  shows  lines,  of  dragged  material,  indicating  the  direction 


DYNAMIC     AND     STRUCTURAL     GEOLOGY 


157 


of  movement.  The  accurate  angle  of  this  direction  of 
scratching  or  dragging  with  a  horizontal  line  drawn  on  the 
fault  plane  may  be  substracted  from  90°  to  give  the  angle 
abc  (Fig.  25).  Then  the  already  found  lateral  separation 
be,  divided  by  the  cosine  of  abc,  gives  the  total  displacement 
ab. 

Of  these  three  functions,  the  perpendicular  separation  Is 
most  easy  of  measurement,  and  its  value  may  vary  from 


Fig.  26.  Stereogram  to  illustrate  the  computation  of  a  fault  movement,  where  a 
part  of  the  data  is  concealed. 


zero  to  the  full  amount  of  lateral  separation.  The  lateral 
separation  is  easier  to  ascertain  than  the  total  displacement, 
and  its  value  may  vary  from  zero  to  the  total  dis- 
placement. In  Fig.  27  a  case  is  illustrated  where 
the  lateral  separation,  the  perpendicular  separation,  and 
the  vertical  separation*  of  the  faulted  beds  are  zero;  but 


'Seep.  160. 


158  GEOLOGY  APPLIED  TO   MINING. 

if  an  ore-body  has  been  faulted  as  represented  in  the 
figure,  then  the  throw  and  the  offset,*  which  in  th's 
case  coincide  with  each  other  and  with  the  total  displace- 
ment, may  be  measured. 


Fig.  27.  Stereogram  of  a  fault  in  which  the  lateral  separation,  the  perpendicular 
separation  and  the  vertical  separation  are  zero. 

Example:  Fig.  28  is  an  ideal  representation  of  that 
class  of  faults  where  the  movement  takes  place 
along  the  bedding  planes  of  stratified  rocks — bedding 
faults.  With  regard  to  the  functions  of  such  a  fault,  it 
will  be  observed  that,  as  far  as  the  stratified  rocks  along 
whose  bedding  the  fault  occurs  are  concerned,  the  fault  has 
no  perpendicular  separation  nor  vertical  separation;  and 
the  other  functions  are  usually  impossible  of  measurement. 


Fig.  28.  Stereogram  illustrating  a  bedding  fault. 

Where,  however,  as  is  represented  in  the  figure,  a  dike  or 
vein  runs  across  the  stratification  and  is  displaced  by  the 
fault,  this  affords  opportunity  for  measuring  the  throw  or 
offset  (which  coincide  in  this  case).  Moreover,  the  perpen- 

*  See  pp.  159,  162. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY  159 

dicular  and  lateral  separation  of  the  dike  may  be  measured, 
and  perhaps  the  total  displacement  may  be  approximately 
calculated  or  directly  measured,  as  for  example,  between 
the  parts  of  the  characteristic  curve  a  (distance  a-a). 


The  measurements  which  have  been  defined  have  no 
constant  direction,  since  they  refer  to  fault  movements 
which  are  capable  of  infinite  variation.  In  general  geo- 
logical work,  however,  it  is  often  only  possible  to  measure 
fault  movements  along  certain  arbitrary  planes.  The 
most  valuable  of  these  planes  are,  the  earth's  surface, 
which  may  be  considered  a  horizontal  plane,  and  vertical 
sections,  into  which  available  data  are  put,  with  the  gaps 
in  the  chain  of  information  often  theoretically  filled  out. 
In  such  cases,  where  some  dislocation  is  evident,  but  the 
formation  is  so  meager  that  it  is  not  possible  to  know  the 
fault  so  accurately  as  to  estimate  even  approximately  its 
total  displacement,  or  lateral  or  perpendicular  separation, 
it  is  necessary  to  employ  specific  terms  to  designate  the 
known  or  estimated  dislocation,  although  the  relations  of 
these  dislocations  to  the  total  displacement  may  be  un- 
known. For  this  purpose  the  terms  offset,  throw  and 
vertical  separation  may  be  used.  The  terms  throw  and 
vertical  separation  are  applied  to  the  dislocation  of  a  fault 
as  seen  in  a  vertical  section;  the  term  offset,  to  the  dislo- 
cation as  seen  in  a  horizontal  section,  such  as  the  earth's 
surface  may  be  considered  to  be. 

The  throw  may  be  defined  as  the  distance  between  the 
two  parts  of  any  body  available  as  criterion  (such  as  a 
sedimentary  bed)  when  these  parts  have  been  separated 


160          GEOLOGY  APPLIED  TO  MINING. 

by  a  fault,  the  distance  being  measured  along  the  fault 
plane,  as  shown  in  a  vertical  section. 

The  vertical  separation  is  the  perpendicular  distance 
between  the  intersection  of  any  two  parts  of  any  faulted 
body  available  as  criterion  (such  as  a  sedimentary  bed), 
with  the  plane  of  a  vertical  section,  the  lines  of  intersection 
being  projected  if  necessary  for  the  purpose  of  measure- 
ment. In  perpendicular  faults  the  vertical  separation  is 
identical  with  the  throw;  in  all  others  it  is  less  than  the 
throw,  but  sustains  a  certain  relationship  to  it,  being  one 
side  of  a  right  triangle  of  which  the  throw  is  the  hypotenuse. 
Thus  the  vertical  separation  may  vary  from  zero  to  the 
full  amount  of  the  throw.  The  throw  is  always  a  part  of 
the  total  displacement,  although  with  no  definite  relation- 
ship to  it,  and  varies  from  zero  to  the  total  displacement. 


Example:  Fig.  29  is  an  ideal  vertical  section  of  faulted 
stratified  rocks;  ab  is  the  vertical  separation,  ac  the  throw. 
Suppose  the  whole  belt  occupied  by  the  fault  covered  from 
observation  in  some  way :  then  the  only  evidence  of  faulting 
which  we  have  would  be  the  fact  that  in  different  places  we 
find  the  same  bed  in  different  positions,  and  if  we  project  the 
different  known  parts  of  this  bed,  they  will  not  meet. 
This  would  be  evidence  of  the  probable  existence  of  a  fault, 
but  we  would  not  know  the  direction,  nor  angle  of  it,  and 
so  would  be  unable  to  measure  the  throw  even  approx- 
imately. We  would,  however,  be  able  to  measure  the 
vertical  separation.  If  the  fault  represented  in  the  dia- 
gram were  perpendicular  to  the  strata,  the  vertical  sepa- 
ration would  coincide  with  the  throw;  if  it  were  horizontal 
the  vertical  separation  would  be  zero. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY 


161 


Fig.  30  represents  the  relations  of  throw  and  vertical 
separation,  more  diagrammatically,  and  in  the  case 
of  a  reversed  fault. 


Fig.  29.  Illustrating  fault  functions. 

The  vertical  separation  being  measured,  the  throw  may 
be  calculated,  if  the  attitude  of  the  fault  is  known;  for  the 


Fig.  30.  The  relations  of  throw  and  vertical  separation,  in  the  case  of  a 
reversed  fault. 

inclination  of  the  fault  (as  shown  in  a  vertical  section) 
from  the  horizontal,  minus  the  dip  of  the  faulted  beds, 


162  GEOLOGY  APPLIED  TO  MINING. 

equals  the  angle  acb  (Fig.  30.)  (fee,  the  dip  of  the  fault, 
equals  ecd,  which,  minus  gcd,  the  dip  of  the  bed,  equals 
acb.}  Then  the  throw  equals  the  vertical  separation 
divided  by  the  sine  of  acb. 


The  term  offset  may  be  used  to  designate  the  perpendicu- 
lar distance  between  the  intersections  of  corresponding 
planes  in  the  two  parts  of  any  faulted  body  available  as 
criterion,  such  as  a  sedimentary  bed,  with  a  horizontal 
plan  such  as  the  earth's  surface  maybe  considered  to  be; 
the  planes  being  projected  for  the  purpose  of  measuring, 
if  necessary.  Like  the  throw,  the  heave  or  offset  is  a  part 
of  the  total  displacement,  but  has  no  definite  relationship 
to  it. 

Example:  Fig.  31  shows  a  horizontal  surface  plan,  com- 
prising a  lake  and  rivers.  The  outcrop  of  the  dotted  bed  is 
displaced  by  the  fault,  and  the  offset  of  the  fault  is  indi- 
cated by'  the  dotted  lines.  If  it  is  desired  to  find  the  dis- 
tance, along  the  outcrop  of  the  fault  plane,  of  the  two  parts 
of  the  bed  separated  by  the  fault  (a  function  which  we  may 
term  the  horizontal  throw),  this  distance  may  be  calcu- 
lated from  the  offset  and  the  direction  of  the  fault  outcrop, 
in  the  same  manner  as  indicated  for  the  vertical  throw  and 
the  vertical  separation. 

To  sum  up,  there  are  six  terms  which  may  designate  the 
different  parts  of  a  fault  movement,  each  term  applying  to  a 
measurement  which  varies  in  accuracy  and  proximity  to 
the  total  displacement  in  proportion  to  the  available 
amount  of  information.  For  general  outline  work  where  ac- 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  163 

curate  data  are  not  obtainable,  the  terms  throw  and  vertical 
separation,  referring  to  the  measurement  of  a  fault  at  its 
intersection  with  a  vertical  plane,  and  the  term  offset,  indi- 
cating the  measurement  of  a  fault  at  its  intersection  with  a 
horizontal  plane,  are  adopted.  The  throw  and  offset  are 
parts  of  the  actual  fault  movement,  but  of  unknown  value, 
while  the  vertical  displacement  sustains  a  certain  relation- 


Fig.  31.  Diagram  illustrating  the  offset  of  a  fault. 

ship  to  the  throw.  Where  more  complete  data  are  obtain- 
able, the  terms  total  displacement,  lateral  separation,  and 
perpendicular  separation  are  adopted.  The  perpendicular 
separation  sustains  a  certain  relationship  to  the  lateral 
separation,  as  the  lateral  separation  does  to  the  total  dis- 
placement. 


164          GEOLOGY  APPLIED  TO  MINING. 

FOLDS  AND  FAULTS  AS  LOCI  OF  ORE-DEPOSITION. 

DEPOSITION  OF  ORE  IN  FOLDS. 
In  what  cases  are  ores  formed  by  preference  in  synclines  or 

anticlines? 

Where  there  is  a  stratum  impervious  to  water  and  that 
stratum  is  -folded  with  others,  downward  moving  waters 


Fig.  32.  Auriferous  Saddle  Veins.    New  Chum  Consolidated  Mine,  Bendigo,  Aus- 
tralia.    C  is  quartz  in  apex  of  saddle.    After  T.  A.  Rickard. 

will  be  arrested  in  the  bottoms   (troughs)   of  synclines 
(downfolds),  and  upward  moving  waters  in  the  tops  of 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  165 

anticlines  (upfolds).  If  the  waters  are  mineralizing,  the 
ores  will  be  deposited  by  preference  in  these  places.  In  a 
given  district  the  chief  mineralization  has  generally  been 
brought  about  in  large  part  either  by  upward  or  by  down- 
ward moving  waters,  so  that  the  ores  may  be  found  either 
in  the  anticlines  or  the  synclines,  as  the  case  may  be;  and 
once  the  law  has  been  discovered  it  is  easy  to  follow.  For 
example,  if  it  is  found  that  the  tops  of  the  anticlines  are 
likely  to  carry  ore,  all  anticlines  must  be  prospected. 


Example:  In  the  Bendigo  gold-fields,  Australia,  aurifer- 
ous quartz  veins  occur  in  highly  folded  Silurian  sandstones 
and  slates.*  The  ore-bodies  are  apt  to  be  especially  large 
and  profitable  at  the  apex  of -anticlines,  forming  so-called 
"saddles"  (Fig.  32),  while  in  synclines  similar  deposits, 
called  "inverted  saddles,"  though  recognized,  are  rare 
and  unimportant. 


Do  folds  need  to  be  pronounced,  in  order  thus  to  determine 
ore-deposition? 

Undoubtedly  a  strong  fold  in  a  relatively  impervious 
stratum  is  more  favorable  than  a  weak  fold  for  producing 
the  localization  of  ores  deposited  by  circulating  waters. 
Yet  a  slight  flexure,  such  as  a  slight  transverse  trough  or 
arch  in  already  highly  folded  and  steeply  dipping  beds, 
may  determine  ore-deposition  and  the  location  of  an  ore 
body. 


*  T.  A.  Rickard,  Transactions  American  Institute  Mining  Engineers,  Vol, 
XX,  pp.  463  et  seq. 


166  GEOLOGY  APPLIED  TO  MINING. 

Example:  The  ores  of  the  Elkhorn  mine,  Jefferson 
county,  Montana,*  lie  on  the  under  side  of  the  contact  of 
limestone  (below)  and  hardened  shale  (above).  These 
strata  dip  35°  to  50°  uniformly  in  the  same  direction, 
forming  part  of  the  main  fold  of  the  region.  In  this  fold 
there  are  several  minor  transverse  corrugations,  forming 
arches  and  troughs.  The  ores  occur  in  two  of  the  lesser 
arches,  which  pitch  steeply,  with  the  general  dip  of  the 
strata  and  unite  near  the  surface  to  form  a  single  broader 
arch.  Along  the  contact  of  limestone  and  hardened  shale 
the  limestone  has  been  crushed  by  slipping  in  the  process  of 
folding.  This  ciushed  rock  formed  the  channel  for  uprising 


Fig.  33.  Occurrence  of  ore  shoots  in  pitching  arches  or  folds  of  the  strata,  Elk- 
horn  mine,  Montana.    After  W.  H.  Weed. 

metalliferous  solutions,  which  were  confined  under  the 
arches  by  the  overlying  relatively  impervious  hardened 
slate,  and  there  the  ore  was  deposited  (Fig.  33). 

Why  do  oil  and  gas  often  occur  at  the  summits  of  anticlinal 

folds? 

The  same  principle  that  arrests  and  accumulates  ascend- 
ing waters  in  the  summits  of  anticlines  holds  good  for  other 
fluids.  Of  great  interest  in  this  respect  are  oil  and  gas, 

*  W.  H.  Weed,  22d  Annual  Report  United  States  Geological  Survey,  Part 
II.  pp.  492-495. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY. 


167 


both  of  which,  forced  upward  under  pressure,  often 
accumulate  under  anticlines,  especially  anticlinal  domes, 
even  if  the  folds  be  very  gentle  and  often  barely  percep- 
tible. Borings  for  oil  or  gas  are  generally  directed  by 
preference  to  these  anticlines. 

Js  this  the  only  reason  why  the  crests  of  anticlines  are  often 

selected  as  sites  of  ore-deposition? 

In  the  process  of  folding  the  tops  of  the  anticlines  are 
the  most  pulled  apart,  the  troughs  of  the  synclines  the 
most  compressed;  hence  at  the  tops  of  the  anticlines 


Fig.  34.  Vein  formation  in  the  apex  of  an  anticline.    New  Chum  Railway  mine, 

Bendigo,  Australia.    M  Nis  apex  of  saddle  occupied  by 

quartz.    After  T.  A.  Rickard. 

there  is  likely  to  be  a  strong  jointing  and  fracturing.  This 
permits  the  passage  of  waters  and  so  determines  a  water- 
course; and  if  the  waters  contain  metals  they  may  be 
precipitated  here. 

Example:  1.  In  many  of  the  saddles  of  auriferous 
quartz  in  the  Bendigo  gold-fields  Australia,  mentioned 
above,  the  vein  penetrates  upward  through  the  beds  along 


168  GEOLOGY  APPLIED  TO  MINING. 

1he  axis  of  the  anticlinal  fold,  in  such  a  way  as  to  indicate 
that  it  has  selected  this  position  on  account  of  the  zone  of 
weakness.  Fig.  34,  showing  an  ore-body  in  the  New 
Chum  Railway  mine,  is  illustrative  of  a  number  of  such 
cases  described  by  Mr.  Rickard. 

2.  The  mining  district  of  Tombstone,  Arizona,  has  as 
rock  formations  a  sedimentary  series  of  limestones, 
quart zites  and  shales,  intruded  by  granodiorite*  and 
overlain  by  rhyolite.  The  sedimentary  rocks  are  also  cut 
by  many  small  dikes  of  granitic  and  dioritic  rocks.  The 
series  has  been  folded,  producing  anticlines,  which  are 
often  highly  compressed,  and  occasionally  faulted;  and  to 


Fig.  35.  Deposition  of  ores  in  anticlinal  folds.    Tombstone  district,  Arizona. 
After  John  A.  Church. 

this  folding  and  fissuring  the  rocks  owe  their  ore.  These 
ores  extend  along  the  stratification,  as  bedded  deposits,  or 
cut  across  it  as  true  veins,  or  again  have  quite  an- irregular 
shape;  and  all  of  these  types  run  into  one  another.  The 
bedded  deposits  and  the  vein§  lie  in  general  in  the  anticlines, 
while  the  synclines  are  barren.  Although  the  evidence 
suggests  that  the  ores  have  been  deposited  from  uprising 
waters,  there  is  no  impervious  stratum  which  has  arrested 
the  upward  passage  of  the  solutions  and  brought  about 
precipitation.  In  one  anticline  there  may  be  as  many  as 
three  separate  sheets  of  ore,  one  over  the  other.f 

*  A  granular  rock  intermediate  in  composition  between  granite  and  diorite. 
t  John  A.  Church,  Transactions  American  Institute  Mining  Engineers,  Vol. 
XXXIII,  pp.  3-37. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  169 

The  result  of  folding  has  been  to  produce  openings  in  the 
anticlines,  both  between  the  strata  and  as  cross-cutting 
fractures;  these  openings  have  constituted  channels  for 
rising  waters,  and  along  them  the  ores  have  been  deposited 
where  favorable  opportunities,  such  as  a  chance  for  replace- 
ment of  the  limestones,  offered  themselves  (Fig.  35). 

DEPOSITION  OF  ORE  ALONG  FAULTS. 
Why  are  ores  often  formed  on  or  near  faults? 

Mineralization  often  takes  place  along  fault  zones  be- 
cause these  have  afforded  the  most  available  circulation 
channels  for  the  mineralizing  waters. 

Example:  An  example  is  found  in  the  Aspen  district, 
Colorado.*  Fig.  36  is  a  section  in  the  Bushwhacker-Park 
Regent  mine,  showing  this  feature.  The  ores  have  chiefly 
formed  along  a  fault  which  makes  only  a  comparatively 
slight  angle  with  the  stratification,  and  especially  at  the 
intersection  of  this  fault  with  others.  Thus  the  ore-bodies 
are  restricted  to  certain  localities  on  the  faults,  while 
other  parts  of  the  faults  are  slightly  or  not  at  all  miner- 
alized. The  reason  for  this  is  partly  because  some  of  the 
faults  originated  subsequent  to  the  chief  period  of  oie- 
deposition;  but  chiefly  because  the  junction  of  two  faults 
made  very  favorable  conditions  for  ore-deposition,  as 
explained  in  the  paragraph  on  the  principle  of  inter- 
sections (See  p.  196). 

Are  the  largest  faults  the  most  favorable  for  ore-deposition? 

The  magnitude  of  the  fault  has  no  relation  to  the  relative 

likelihood  of  ore-deposits,  for  the  favorable  circumstance  is 

*  Monograph  XXI,  United  States  Geological  Survey,  pp.  229-231. 


170 


GEOLOGY  APPLIED  TO  MINING. 


the  fissuring  and  crushing,  producing  channels  of  circula- 
tion, and  not  the  fault  movement.  Thus  a  very  slight 
fault  may  be  far  more  thoroughly  mineralized  than  a  large 
one. 


Fig.  36.   Cross-section  of  Bushwhacker-Park  Regent  mine,  Aspen,  Colorado. 

Dark-shaded  areas  are  ore-bodies.    Heavy  black  lines  are 

faults.    After  J.  E.  Spurr. 

Why,  in  a  faulted  country  are  the  ore-bodies  irregular  and 
why  do  they  often  form  rather  near  the  faults  than  on  them? 
A  fault  is  generally  not  a  single  plane;  it  is  a  zone  of 

close-set  fractures,  the  movement  being  most  intense  along 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  171 

a  certain  line  and  dying  out  slowly  on  both  sides.  Fre- 
quently the  rock  on  both  sides  of  the  fault-zone  is  thoroughly 
wrenched  and  seamed  with  tiny  cracks,  even  where  it 
appears  solid  to  the  naked  eye.  The  mineral  solutions  are 
more  effective  among  slight  fractures  than  in  a  large  fissure; 
thoroughly  seamed  rock  is  a  very  favorable  place  for  ore, 
because  the  solutions  are  checked  and  held  in  a  way  that 
seems  fitted  for  the  working  of  the  reactions  which  lead  to 
the  precipitation  of  ores.  If  mineralization  is  slight  along 
the  main  fault  fracture  it  may  be  considerable  along  some 
of  the  auxiliary  fractures,  and  in  the  strained  rock  near  by. 
This  is  especially  the  case  in  limestones,  where  great 
deposits  thus  originate.  Therefore  the  search  for  minerali- 
zation along  a  fault  plane,  in  districts  where  the  two  are 
associated,  should  extend  over  a  comparatively  wide  zone. 

Example:  1.  The  veins  of  Rico,  Colorado,  as  described 
by  T.  A.  Rickard  and  F.  L.  Ransome,  are  mostly  along 
fissures  which  have  been  opened  by  faulting.  The  dis- 
placement of  these  faults,  as  shown  on  each  side  of  the 
veins,  is,  however,  generally  less  than  10  feet.  As  a  rule, 
the  more  important  faults  of  the  region  are  not  attended 
by  much  ore.  The  ore-bearing  fault  is  often  a  slight  aux- 
iliary slip,  occurring  beside  the  main  plane  of  movement 
of  a  larger  fault,  which  is  barren. 

The  result  of  dynamic  strain  in  this  region  was  the 
development  of  planes  or  zones  of  weakness,  cracks  and 
fissures;  and  along  these  there  was  generally  movement  of 
the  rock  on  one  side  past  the  rock  on  the  other.  All  of 
these  openings  became  the  channels  of  circulating  mineral- 
bearing  waters.  In  the  smaller  channels,  however,  (which 
naturally  were  along  the  smaller  faults)  the  circulation 


172 


GEOLOGY  APPLIED  TO  MINING. 


Sandstone.    Sandy  Limestone.  Bhodo-       Quartz.        Zinc       Crushed 
Lime.  chrosite.  Blende.       Rock. 

Fig.  37.  Ore  deposition  in  fissure  along  a  minor  fault.    Section  across  the  Eureka 
vein,  Rico,  Colorado.    After  T.  A.  Rickard. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY. 

was  slow,  so  that  plenty  of  time  was  given  for  the  processes 
of  deposition  to  act;  while  in  the  larger  channels  the  circu- 
lation was  probably  in  many  cases  so  rapid  as  not  to  allow 
much  precipitation  along  them.  Fig.  37  shows  a  fissure 
vein  along  a  minor  fault. 

2.  In  the  Queen  of  the  West  mine,  Ten-Mile  district, 
Colorado,  are  sandstone  and  shale  beds,  with  generally 
conformable  porphyry  sheets.  Faulting  has  taken  place 
along  a  series  of  parallel  and  closely  contiguous  planes,  so 
that  the  rock  has  been  divided  into  thin  sheets,  each  of 
which  has  moved  past  the  other  a  certain  distance.  In 
the  central  part  of  the  fissured  zone  the  spaces  between  the 
sheets  have  been  filled  with  vein  material,  and  the  sheets 
themselves  decomposed,  impregnated,  and  somewhat 
replaced  by  it.  The  resulting  condition  is  puzzling  for  the 
miner  who  expects  to  find  his  ore  bounded  by,  well-defined 
walls.  There  are  here  walls  in  abundance,  but  no  one  wall 
can  be  followed  continuously  for  any  great  distance. 
Therefore  it  is  the  custom  to  run  frequent  cross-cuts  away 
from  the  main  drift  (which  follows  the  central  zone)  and 
these  cuts  disclose  ore-bodies  running  parallel  to  this  zone, 
now  on  one  side  and  now  on  the  other,  and  often  15  or  25 
feet  distant  from  it.* 

JOINTS  IN  ROCKS. 

What  are  joints  in  rocks? 

Joints  are  planes  of  fracture,  or  divisional  planes,  which 
run  through  rocks.  Few  rocks  are  without  them.  Frag- 
ments of  broken  rocks  are  often  more  or  less  completely 


*  S.  F.  Emmons,  Transactions  American  Institute  Mining  Engineers,  Vol. 
XVI,  p.  837. 


174  GEOLOGY  APPLIED  TO  MINING. 

bounded  by  plane  surfaces,  whether  the  rock  is  igneous  or 
stratified.  In  stratified  rocks  one  or  two  of  the  plane 
surfaces  are  apt  to  be  due  to  the  stratification;  the  others 
are  joints.  In  igneous  rocks  all  the  planes  are  usually 
joints. 

How  are  joints  produced? 

Joints  are  produced  by  the  application  of  force  to  the 
rock.  Earth  movements  may  cause  a  strain  or  twisting — 
the  result  is  a  system  of  cracks,  such  as  we  find  when  ice 
or  glass  is  put  under  such  strains.  According  to  the 
nature  of  the  stress,  the  number  of  joint  systems  vary, 
together  with  their  general  direction  and  their  direction 
relative  to  one  another,  and  the  relative  abundance  of 
joint  cracks  in  the  rock. 


Fig.  38.  Columnar  jointing  of  basalt,  Koyukuk  mountain,  on  the  Yukon  river, 
Alaska,    After  J.  E.  Spurr.* 


*  18th  Annual  Report  United  States  Geological  Survey,  Part  III. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  175 

Are  all  joints  formed  by  regional  strains? 

Another  kind  of  joint  is  formed  by  contraction.  This 
is  found  in  most  lavas  and  in  many  small  dikes.  The 
heated  rock  shrinks  and  cracks  on  cooling.  The  resulting 
jointing  is  usually  systematic;  it  runs  in  the  direction  of 
least  resistance  and  hence  is  vertical  to  the  planes  of  the 
flow  in  lavas,  and  perpendicular  to  the  walls  in  dikes. 
Its  effect  is  to  divide  the  rock  into  columns — hence  the 
term  columnar  structure  (Fig.  38).  This  columnar  joint- 
ing may  also  originate  by  reason  of  rocks  shrinking  through 
chemical  changes. 

ORE-DEPOSITION  ALONG  JOINTS. 

What  advantage  is  there  to  the  mining  man  in  the  study  of 

rock-joints? 

Joint-planes  have  nearly  uniform  directions  for  long 
distances,  traversing  even  folded  beds.  Since  they  are  the 
record  of  strains  in  the  earth's  crust,  their  study  should 
never  be  neglected  by  the  student  of  mining  geology. 
Frequently  there  is  an  observable  connection  between  them 
and  fold  and  fault  systems,  mountain  ranges,  etc.,  in  the 
same  district.  Moreover,  joint  planes,  especially  when 
closely  set  together,  furnish  a  channel  for  underground 
waters.  Hence  the  joint  systems  may  correspond  to  the 
vein  systems  in  a  given  district,  and  a  study  of  the  former 
helps  in  exploring  and  exploiting  to  best  advantage  the 
latter. 

Example:  The  mining  camp  of  Monte  Cristo,  in  the 
Cascade  Range,  Washington,  is  in  a  district  where  ninety- 


176 


GEOLOGY  APPLIED  TO  MINING. 


nine  per  cent,  of  the  ores  have  formed  along  joint-planes. 
These  planes  have  furnished  channels  for  circulating 
waters  and  as  a  consequence  the  minerals  which  these 
waters  carried  have  been  deposited  near  the  joints  (Fig.  39). 
A  study  of  the  laws  of  jointing  here  is  directly  applicable 
to  the  mineral  veins,  for  every  peculiarity  of  the  jointing  is 
copied  by  veins,  with  the  added  complication  that  the  vein 
(following  the  former  or  even  present  channel  of  easiest 
circulation  for  waters)  may  pass  from  one  joint  to  another. 


SCALE  Or  FEET 

5 


Fig.  39.  Formation  of  ores  along  joints.    Tunnel  and  vein  exposures  in  vertical 
cliff,  Glacier  creek,  Monte  Cristo,  Washington.    After  J.  E.  Spurr. 


How  should  one  study  joints  so  as  to  arrive  at  an  under- 
standing of  the  system? 

The  strikes  and  dips  of  joints  in  various  places  should  be 
recorded  on  the  map  by  the  same  symbol  given  for  strati- 
fied rocks;  an  accumulation  of  these  records  and  their 
combination  usually  enables  one  to  comprehend  the  joint- 
systems. 


DYNAMIC     AND    STRUCTURAL    GEOLOGY.  177 

FRACTURES  AND  FISSURES. 

What  is  meant  by  the  term  fractures  as  used  in  mining  geology  f 
The  term  fractures  is  generally  applied  to  cracks  in  rocks, 
large  enough  to  be  distinctly  visible  to  the  naked  eye. 
The  fractures  may  also  come  under  the  head  of  joints,  in 
which  case  they  are  joint  fractures  or  joint  cracks;  or,  if 
there  has  been  movement  along  them,  they  may  also  be 
faults  and  fault  fractures.  As  generally  used,  the  term 
denotes  a  break  of  an  importance  intermediate  between  a 
joint  and  a  fault,  as  these  latter  are  most  commonly 
employed. 

Fractures  are,  like  joints  and  faults,  the  result  of  the 
straining  and  cracking  of  rocks  under  pressures.  A  fault 
plane  is  usually  accompanied  on  both  sides  by  parallel 
fractures  extending  a  greater  or  less  distance  away  from  it, 
and  there  may  or  may  not  be  faulting  along  them.  In 
regions  where  the  rocks  have  been  jointed  by  stress,  one  or 
several  of  the  systems  may  be  so  strongly  developed  as  to 
cause  marked  fractures,  often  a  conspicuous  feature  in  the 
general  appearance  of  the  rocks. 

In  what  way  is  this  stress  applied  so  as  to  form  fractures  and 

fissures? 

According  to  the  way  motion  takes  place  in  the  crust, 
certain  portions  may  be  stretched  or  compressed.  By 
both  these  methods  fractures  and  fissures  may  originate; 
in  the  former  by  the  production  of  actual  cracks  from  the 
stretching;  in  the  latter  by  the  greater  action  of  the  com- 
pressive  strains  along  certain  lines,  there  producing  zones 


178          GEOLOGY  APPLIED  TO  MINING. 

of  more  considerable  crushing,  shearing  and  faulting.  It 
is  certain  that  open  cracks  and  fissures  may  be  formed 
directly  from  stretching  (tension) ;  while,  from  the  nature 
of  things,  it  is  mpossible  that  openings  are  directly  pro- 
duced by  compression.  In  the  case  of  fracture  zones 
resulting  from  compression,  however,  release  of  the  pressure 
may  allow  them  to  open  somewhat;  they  then  become  the 
channels  for  circulating  waters,  and  these  may  dissolve  or 
otherwise  carry  away  the  broken  or  ground-up  material, 
leaving  a  continuous  opening  or  series  of  openings. 

Twisting  or  torsion  of  a  rigid  body  has  been  experi- 
mentally found  to  produce  systems  of  cracks  intersecting  at 


Fig.  40.  Sheet  of  glass  cracked  by  torsional  strain.    After  Daubree. 

right  angles.  The  force  in  this  case  seems  to  be  tensional 
(Fig.  40).  This  process  probably  takes  place  in  the  earth's 
crust. 

Earthquake  shocks  probably  produce  shattering  and 
fracturing  of  the  rocks.  This  also  is  a  sort  of  tensional 
stress. 

Eruptions  and  intrusions  of  volcanic  material  may  cause 
fracturing  and  fissuring  of  the  rigid  rocks  through  which 
they  pass,  especially  in  the  general  neighborhood  of  the 
surface. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  179 

What  are  conjugated  fractures,  and  how  are  they  formed? 

It  has  been  shown  experimentally  and  by  calculation 
that  compressive  stress,  applied  horizontally  in  a  given 
direction  (for  instance,  from  north  to  south),  will  produce 
two  systems  of  fractures,  striking  east  and  west,  and 
dipping. respectively  north  and  south  at  angles  of  about 
45°.  These  two  systems  of  fractures,  parallel  in  strikes 
but  opposite  in  dip,  are  designated  conjugated  fractures. 

Are  fractures  straight,  or  irregular? 

As  a  rule,  fractures,  like  joints  and  faults,  being  due  to 
stress,  are  straight,  approaching  as  near  as  possible  mathe- 
matical planes.  In  rocks  which  are  uniform  in  texture 
and  resistance,  therefore,  fractures  generally  deviate  but 
little  from  a  straight  course;,  but  as  nearly  all  rocks  are 
more  or  less  irregular  in  these  particulars,  small  and  even 
large  irregularities  will  be  found  in  actual  fractures.  The 
same  is  true  in  fault  planes. 


Example:  In  the  Mercur  mine,  Mercur  district,  Utah, 
there  is  a  system  of  open  cracks  and  fissures,  cutting  the 
limestones  and  also  traversing  a  system  of  calcite  veins, 
later  in  age  than  the  limestone,  but  earlier  than  the  fissures. 
These  cracks  often  follow  the  course  of  a  calcite  vein, 
which  evidently  offered  less  resistance  than  the  limestone. 
Even  where  the  vein  and  the  fissure  are  perpendicular  one 
to  the  other,  the  latter  is  often  deflected  by  the  former 
(Fig.  41).  The  accompanying  sketch  is  from  the  wall  of  a 
tunnel  in  the  Mercur  mine. 


180 


GEOLOGY  APPLIED  TO  MINING. 


Again,  the  visible  crack  which  we  call  a  fracture  may  be 
a  combined  series  of  joints  or  fractures,  each  approaching  a 


JLimestone. 

qpen  fissure. 

Fig.  41.  Open  flssure  cutting  and  deflected  by  calcite  vein,  Mercur  mine,  Utah 
After  J.  E.  Spun-.* 

mathematical  plane,   but  together  having   an   irregular 
course. 

*  16th  Annual  Report  United  States  Geological  Survey,  Part  II,  p.  409. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  181 

How  do  fractures  behave  in  different  stratified  rocks? 

In  non-homogeneous  rock,  as  has  been  pointed  out, 
fractures,  from  whatever  initial  stress  they  may  arise,  will 
tend  to  be  deflected  in  the  direction  of  least  resistance.  In 
thin  beds  of  homogeneous  rock,  such  as  a  dense  sandstone 
or  limestone,  this  direction  is  apt  to  be  directly  across  the 
bed,  perpendicular  to  the  stratification. 

In  a  shale,  on  the  other  hand,  the  fracture  will  tend  to 
be  deflected  in  the  direction  of  the  bedding,  and  in  slates 
in  the  direction  of  the  cleavage.  A  fracture  may  entirely 
cease  on  encountering  a  transverse  fracture,  the  move- 
ment being  deflected  and  taken  up  by  the  latter. 

Fractures  are  naturally  most  clean  cut  and  persistent 
in  rigid  rocks,  like  quartzites  and  igneous  rocks.  In  soft 
rocks  like  shales,  any  movement  arising  from  stress  may 
be  partly  or  wholly  taken  up  by  the  yielding  of  the  rock 
near  the  disturbance,  a  giving  way  analogous  to  flowage. 

Therefore,  in  passing  from  one  stratum  to  another  such 
as  from  a  sandstone  into  a  shale,  fractures  often  change 
both  in  direction  and  intensity.  A  strong  fracture  in  a 
sandstone  bed  may  die  out  entirely  in  passing  into  a  shale. 

Are  fractures  and  joints  always  persistent  even  in  rigid  and 
homogeneous  rocks? 

Fractures,  joints  and  faults  may  become  less,  and  even 
die  out  in  rigid  strata,  where  there  is,  nevertheless,  enough 
yielding  progressively  to  take  up  more  and  more  of  the 
dislocation. 


182  GEOLOGY  APPLIED  TO  MINING. 

What  are  imbricating  fractures? 

It  frequently  happens  that  when  a  fracture  stops,  the 
continuation  of  the  same  line  of  weakness  is  shown  by  a 
parallel  fracture,  a  little  to  one  side,  beginning  near  where 
the  other  leaves  off,  or  overlapping.  These  may  be  called 
imbricating  fractures. 

How  are  open  fissures  formed  near  the  surface? 

Even  in  greatly  disturbed,  folded,  faulted  and  jointed 
rocks,  open  fissures,  larger  than  cracks,  are  relatively  rare 
and  unimportant.  At  the  very  surface,  the  shrinkage  of 
volume  in  rocks,  resulting  from  chemical  changes,  and  the 
falling  apart  under  the  influence  of  gravity,  cause  many 
fissures.  But  these  conditions  are  characteristic  only 
near  the  surface.  The  rock  encountered  in  mines  is  much 
more  solid. 

Example:  The  granite  rocks  of  Cape  Ann,  Massachusetts, 
especially  as  exposed  along  the  shore,  are  traversed  by 
many  open  joint  fissures  and  by  joint  fractures  (Fig.  42). 
At  a  very  little  distance  below  the  surface,  however,  the 
fissures  disappear,  and  the  fractures  diminish  greatly  in 
frequency,  so  that  the  rock  found  in  quarries  is  a  good 
building-stone. 

How  are  openings  below  the  surface  formed,  and  to  what  depth 

do  they  persist? 

Fissures  encountered  in  mines  are  not  usually  produced 
directly  by  dynamic  action,  but  are  due  to  the  dissolving 
action  of  underground  water  circulating  along  faults, 
fractures,  or  shear-zones.  In  easily  soluble  rocks,  like 


DYNAMIC     AND     STRUCTURAL     GEOLOGY. 


183 


184          GEOLOGY  APPLIED  TO  MINING. 

limestones,  openings  made  in  this  way  are  especially  large, 
and  a  series  of  irregular  connecting  caves  may  result.  In 
less  soluble  rocks,  solution  will  generally  not  produce  caves, 
but  only  irregular  widenings  of  the  original  crevice;  the 
resulting  water  channels  will  be  straighter  than  in  soluble 
rocks,  but  will  open  out  at  one  place,  and  contract  almost 
to  nothing  at  another.  In  the  case  of  the  soluble  rocks, 
open  spaces  are  due  almost  wholly  to  solution;  in  the  case 
of  the  difficultly  soluble  ones,  the  same  may  be  the  case; 
but  in  neither  would  the  water  have  ordinarily  been  able 
to  gain  access,  so  as  to  accomplish  its  dissolving,  without 
some  preliminary  channel  due  to  rending.  One  can  put 
it  as  an  almost  invariable  rule,  therefore,  that  fissures  are 
not  open  and  regular  for  long  (distances.  They  are  rather  a 
string  of'  connected  openings  of  limited  extent.  This  is 
necessarily  true,  for  a  regular  open  fissure  any  distance 
underground  would  soon  be  closed  by  the  effects  of  gravity. 
Irregular  openings,  with  buttresses  of  solid  rock  between, 
supporting  the  weight  cfn  both  sides,  can  and  do  remain 
open  to  considerable  depths.  At  a  certain  ultimate  depth, 
however,  it  is  supposed  that  the  great  pressure  (com- 
bined with  increased  fluidity  of  the  rock,  due  to  increase 
of  temperature)  is  sufficient  to  close  even  openings  of  this 
sort. 

DEPOSITION  OF  ORES  ALONG  FRACTURES  AND  FISSURES. 

What  is  the  application  of  the  study  of  fissures  and  fractures 
to  the  study  of  mineral  veins? 

Fractures  and  fissures  become  the  channels  for  circulating 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  185 

waters,  and  the  seats  of  vein  formation.  Each  one  of  their 
characteristics,  therefore,  is  characteristic  also  of  a  certain 
class  of  mineral  veins. 

Do  the  veins  in  a  given  region  ever  follow  definite  systems  in 
regard  to  their  trend? 

Fractures  and  fissures,  it  has  been  pointed  out,  often 
result  from  a  regional  strain  in  the  crust,  affecting  large 
areas  alike.  They  are,  therefore,  formed  in  definite  sys- 
tems, and  where  they  become  mineralized,  the  veins  fall 
into  similar  groupings. 

Example:  In  the  southwest  of  England,  a  series  of 
fissures  running  north  and  south,  or  north-northwest  and 
south-southeast,  traverses  another  series  which  runs  in  a 
more  east-and-west  direction.  The  latter  in  Cornwall 
contains  the  chief  copper  and  tin  ores,  while  the  former 
contains  lead  and  iron.  The  east-and-west  veins  in  the 
west  part  of  the  region  were  formed  before  those  that  cross 
them,  for  they  are  shifted,  and  their  contents  are  broken 
through  by  the  latter.* 

What  bearing  has  the  irregular  course  of  fractures  on  mineral 

veins? 

Mineralizing  waters  follow  along  fractures  in  all  their 
deviations.  Along  their  course  they  often  deposit  ore, 
both  in  the  fractures  and  in  the  rock,  by  cavity-filling, 
replacement  or  interstitial  filling  (impregnation). 

*  De  La  Beche,  'Geological  Observer,'  p.  659. 


186  GEOLOGY  APPLIED  TO  MINING. 

Example:  Many  of  the  ore-bodies  in  the  Tintic  district, 
Utah,*  furnish  excellent  examples  of  veins  formed  along 
circulation  channels  offered  by  successive  fractures  running 
in  different  directions.  The  accompanying  figure  is  from 
the  Ajax  mine  in  that  district.  The  country  rock  is  lime- 
stone, and  the  ores  (which  consist  chiefly  of  pyrite,  galena, 
and  enargite,  carrying  gold  and  silver,  oxidation  products 
of  these  sulphides,  and  quartz  and  barite  as  gangue  miner- 
als) have  been  deposited  as  replacements  of  this  rock,  in 
the  neighborhood  of  the  fractures.  The  fractures  as  a  rule 
are  continuous  past  the  point  where  they  cease  to  be  ore- 
bearing,  though  this  is  not  well  represented  in  the  diagram 
(Fig.  43).  This  type  of  vein  is  an  important  and  common 
one,  and  grades  into  very  irregular  ore-bodies. 


Does  the  peculiar  behavior  of  fractures  and  joints  in  different 
stratified  rocks  find  an  analogous  behavior  in  mineral 
veins? 

Mineral  veins  in  different  kinds  of  stratified  rocks  show 
a  variation  which  corresponds  exactly  to  the  variations  of 
fractures  under  like  circumstances. 


Example:  In  the  Bendigo  gold-fields,  Australia,  the 
auriferous  quartz  veins,  largely  in  sandstone  and  shale, 
show  many  irregularities  illustrating  these  points.  Fig.  44 
shows  the  cross-section  of  an  open  cut  behind  the  Victoria 
Quartz  mine  in  this  region.  Here  is  seen  how  veins  in  the 
standstone  stop  abruptly  on  reaching  the  shale;  how 
others,  stronger,  persist  into  the  shale,  but  are  deflected  in 


*  G.  W.  Tower,  Jr.,  and  G.  O.  Smith,  19th  Annual  Report  United  States 
Geological  Survey,  Part  III,  p.  724,  et  seq. 


DYNAMIC     AND    STRUCTURAL    GEOLOGY.  187 


2  a 


188 


GEOLOGY  APPLIED  TO  MINING. 


the  direction  of  the  bedding,  and  finally  die  out.  The 
deflections  of  veins  in  sandstone,  passing  through  a  slate 
bed,  is  further  shown  in  Fig.  45,  from  the  same  district.* 


Fig.  44.  Open  cut  of  Victoria  Quartz  mine,  Bendigo,  Australia,      a,  slate;  6, 
sandstone;  c,  gold  quartz  veins.    After  T.  A.  Rickard. 


V, 


7 


j.  45.  Quartz  veins  in  Confidence  Extended  mine,  Bendigo,  Australia. 
After  T.  A.  Eickard. 


*  T.  A.  Rickard.     '  The  Bendigo  Gold  Field,'  (Second  paper).     Transac- 
tions American  Institute  Mining  Engineers,  Vol.  XXI,  pp.  686-713. 


DYNAMIC     AND    STRUCTURAL    GEOLOGY.  189 

Does  the  increase  in  the  number  and  size  of  cracks  and  fissures 
close  to  the  surface  have  an  influence  upon  mineral  veins? 
The  cracks  and  fissures  near  the  surface,  opened  up  in 
the  manner  previously  described,  very  frequently  become 
the  channels  of  mineralizing  waters,  and  the  sites  of  ore- 
deposition,  and  are  thus  transformed  into  mineral  veins. 
Such  veins,  following  the  characteristics  of  the  joints, 
fractures  or  fissures  along  which  they  were  formed,  will  be 
largest  and  most  numerous  at  the  surface.  Below  the 
surface  they  will  become  less  in  number,  and  will  tend  to 
unite,  forming  a  smaller  number  or  even  a  single  well- 
defined  strong  vein,  which  persists  to  a  considerable  depth. 
This  occurrence  is  a  matter  of  common  observation  among 
miners,  who  often  remark  that  a  vein  is  "all  broken  up" 
near  the  surface,  and  that  it  will  get  "more  regular  as  it 
goes  down." 

Under  what  conditions  are  branching  surface  veins,  like  those 

described,  formed? 

It  follows,  from  the  method  of  origin  of  the  fractures  in 
which  the  veins  were  deposited,  that  the  ores  were  brought 
to  their  position  and  there  laid  down  at  a  very  slight 
distance  from  the  surface.  The  waters  which  are  active 
close  to  the  surface  are  descending  atmospheric  waters,  and 
in  a  case  of  this  kind  it  is  usually  these  which  have  formed 
the  veins.  The  fewer  and  more  regular  veins  attained  in 
depth  are  frequently  the  product  of  an  earlier  period  of 
deposition,  which  the  surface  waters  have  worked  over  and 
re-deposited  to  form  the  surface  ores,  as  will  be  explained 
in  the  next  chapter. 


190          GEOLOGY  APPLIED  TO  MINING. 

What  are  filled  deposits? 

Veins  or  ore-bodies  deposited  in  pre-existing  cavities, 
(not  microscopic),  whether  caused  by  rending  or  solution 
(generally  by  both)  may  be  called  filled  deposits. 

What  is  crustification? 

Crustification  is  a  banded  structure  produced  by  suc- 
cessive deposition  of  different  layers  on  the  walls  of  an 
opening;  it  is  often  visible  in  filled  deposits. 

Do  all  filled  deposits  show  crustification? 

Many  deposits  which  have  formed  in  open  cavities  show 
no  banding,  but  an  irregular  arrangement  of  minerals,  or 
are  massive  and  homogeneous  throughout. 

7s  all  banding  in  veins,  parallel  to  the  walls,  crustification? 

Replacement  deposits  may  often  show  banding  in  all 
degress  of  perfection,  sometimes  simulating  almost  exactly 
the  crustification  of  filled  deposits.  This  arises  from  the 
existence  of  bands  of  different  texture  or  chemical  compo- 
sition in  the  original  rock.  Certain  of  the  bands  may 
induce  the  formation  of  a  particular  mineral  during  the 
process  of  replacement,  or  at  any  rate  may  cause  differences 
of  texture,  even  if  the  minerals  deposited  in  the  different 
bands  be  approximately  the  same.  Along  fracture-zones 
or  shear-zones  a  very  perfectly  banded  ore-deposit  may 
form  by  replacement,  for  the  parts  along  the  fracture 
planes  are  replaced  first,  and  afterward,  more  slowly  and 
under  different  conditions,  the  rock  space  between  the 
planes.  This  results  in  either  physical  or  chemical  differ- 


DYNAMIC     AND    STRUCTURAL    GEOLOGY.  191 

ences,  which  are  plainly  visible  as  bands.  Again,  the 
fracture  crevices  may*  be  filled  with  vein  matter,  and  the 
sheets  of  rock  between  may  be  mineralized  by  the  replace- 
ment process;  and  thus  a  banded  appearance  results. 

Example:  According  to  S.  F.  Emmons  there  is  in  many 
of  the  ore-deposits  in  the  Gunnison  region,  in  Colorado,  a 
noteworthy  appearance  of  banded  structure  parallel  with 
the  walls.  Yet  the  evidence  of  thin  sheeting  of  the  country- 
rock  is  so  clear  that  it  is  probable  this  appearance  arises 
from  the  fact  that  some  of  the  bands  are  the  filling  of 
narrow  fissures,  and  others  a  replacement  of  thin  sheets  of 
the  country-rock,  the  differing  composition  of  the  bands 
resulting  from  the  variation  in  the  process  of  deposition. 

Ribbon  structure,  described  on  p.  199,  as  a  sheeting  pro- 
duced parallel  to  the  walls  of  a  vein,  by  movement  subse- 
quent to  its  formation,  may  also  be  mistaken  for  crustifi- 
cation. 

What  is  a  fissure  vein? 

The  best  type  of  the  filled  deposit  is  the  fissure  vein, 
which  may  or  may  not  show  crustification.  Such  a  vein  is 
characterized  by  regular,  straight  walls,  by  a  fairly  constant 
width,  and  by  a  definite  direction  of  both  strike  and  dip. 
There  is  usually  a  sharp  line  of  division  between  the  vein 
and  the  wall  rock,  such  as  is  generally  wanting  in  replace- 
ment deposits. 

Do  ore-deposits  in  pre-existing  cavities  always  form  a  distinct 
class  from  other  deposits? 
Ore-deposits  which  have  filled  pre-existing  cavities  may 


192 


GEOLOGY  APPLIED  TO  MINING. 


also  extend  into  the  wall  rock  of  the  fissures,  by  replace- 
ment or  impregnation,  and  often  no  line  can  be  drawn 
between  the  ores  formed  by  one  process  and  those  formed 
by  another,  the  two  sorts  forming  a  continuous  body. 

What  are  linked  veins? 

Linked  veins  are  the  filling  of  a  series  of  branching  and 
reuniting  fractures,  of  a  peculiar  type  which  can  be  best 
shown  by  illustration  (Fig.  46). 


Fig.  46.  Linked  veins.    Surface  plan  of  vein  system  of  Pachuca,  Mcx.cu. 
Scale  1=50,000.    After  E.  Ordonez. 


Example:  In  the  mining  district  of  Pachuca,  in  Mexico, 
the  veins  follow  a  general  east-west  course,  and  are  united 
by  diagonal  branches.  The  peculiar  character  of  each 
branch  is  that  it  never  crosses  the  veins  that  it  unites. 


DYNAMIC     AttD    STRUCTURAL    GEOLOGY.  193 

Often  two  branches  start  from  a  vein  at  the  same  point  and 
run  in  opposite  directions,  so  that  one  is  apparently  the 
prolongation  of  the  other;  this  circumstance  has  led  some 
miners,  to  the  belief  in  the  crossing  of  the  branches,  and 
has  led  them  into  serious  mistakes.* 

SHEAR  ZONES  OR  CRUSHED  ZONES,  AND  THEIR 
SUITABILITY  FOR  ORE-DEPOSITION. 

What  are  shear  zones  or  fracture  zones,  and  what  influence 

have  they  on  ore-deposition? 

When  a  rock  mass  is  put  under  pressure  by  earth  move- 
ment, some  parts  of  the  rock,  being  weaker,  will  yield  and 
will  be  crushed,  bent  and  broken  to  a  greater  extent.  These 
areas  are  apt  to  be  fairly  regular,  and  generally  they  form 
pretty  well  denned  zones  of  variable  thickness,  often  with 
obscure  walls  between  them  and  the  more  solid  rock.  If 
there  has  been  movement  along  such  a  crushed  zone,  so 
that  the  rock  on  one  side  has  changed  position  noticeably 
with  the  rock  on  the  other  side,  it  becomes  a  fault  or  a 
fault  zone.  Often,  however,  there  is  hardly  any  noticeable 
faulting,  and  in  this  case  we  may  call  it  a  shear  zone,  if  the 
rock  has  been  crushed  and  sheared,  or  a  fracture  zone,  if 
it  has  only  been  especially  intensely  fractured.  In  either 
case  such  a  disturbed  area  or  zone  offers  a  channel  for 
circulating  waters,  and  is  very  favorable  to  ore-deposition, 
for  the  comparative  slowness  of  circulation  enforced  by 
the  obstructed  passage  makes  the  percolation  thorough, 
and  offers  every  chance  for  precipitation. 

*  E.  Ordonez,  Boletin  del  Institute  Geologico  de  Mexico,  'El    Mineral  de 
Pachuca,'  p.  57. 


194  GEOLOGY  APPLIED  TO  MINING. 

Example:  The  gold-quartz  veins  of  Otago,  New  Zealand, 
described  by  T.  A.  Rickard*,  have  formed  largely  in  shear 
zones,  and  the  lodes  show  every  variation  from  a  condition 
where  the  country  rock  (schist)  forms  the  greater  part,  to 
the  entirely  replaced  stage,  where  the  vein  is  clear  aurif- 
erous quartz.  They  are  found  in  channels  but  little 
divided  from  the  main  mass  of  the  country  rock,  and  the 
schists  themselves,  beyond  the  lode  boundaries,  are  often 
auriferous.  Probably  certain  belts  of  the  schist,  outside 
of  the  lodes,  are  sufficiently  mineralized  to  become  mines. 

GENERAL  RELATION  BETWEEN  ROCK   DIS- 
TURBANCES AND  ORE-DEPOSITS. 

Are  regions  of  undisturbed  rocks  favorable  for  ore-deposits? 

In  general  a  region  of  flat  unfolded  rocks  is  poor  in  ore- 
deposits,  as  for  example  the  region  lying  between  the 
Rocky  Mountains  and  the  Appalachians,  as  compared  with 
the  folded  region  lying  between  the  Rocky  Mountains  and 
the  Pacific.  Where  ore-deposits  do  occur  in  such  a  flat 
region,  they  will  often  be  found  to  be  connected  with  some 
minor  disturbance. 

Example:  In  southern  Missouri  and  adjacent  parts  of 
Kansas  and  Arkansas,  the  flat  Paleozoic  strata,  together 
with  underlying  ancient  crystalline  rocks,  have  been 
affected  by  a  monoclinal  uplift,  elliptical  in  outline, 
known  as  the  Ozark  uplift.  On  the  summit  the  bedding 
planes  are  horizontal,  while  throughout  the  border  areas 
they  are  inclined  away  from  the  center.  This  disturbance 
has  produced  fracturing,  more  pronounced  along  the 


*  Transactions  American  Institute  Mining  Engineers,  Vol.  XXI,  pp  411-442. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  195 

borders  than  on  the  tops,  and  the  principal  mining  local- 
ities (producing  lead  and  zinc  ores)  are  situated  around 
this  border  in  such  a  way  as  to  indicate  that  the  mineral- 
ization has  been  dependent  upon  the  fracturing.  In  this 
case  the  mineralization  is  thought  to  have  been  brought 
about  by  descending  waters,  and  not  to  have  been  con- 
nected with  igneous  rocks  or  hot  springs.* 

What  is.  the  reason  for  the  general  connection  of  ore-deposits 

with  disturbed  rocks? 

Mountains,  igneous  rocks,  folded  strata,  hot  springs,  and 
ore-deposits  are  often  all  connected.  The  zones  of 
folding  in  strata  lie  along  certain  lines  of  weakness  in 
the  crust.  The  relief  of  pressure  caused  by  the  giving 
away  of  the  strata  in  the  folded  region  may  cause  a  migra- 
tion of  the  suppressed  molten  rock  beneath  the  crust  to 
this  zone;  eruptions  and  intrusions,  accompanied  by 
further  disturbances,  follow.  Fractures  and  fissures  are 
formed;  by  the  influence  of  the  igneous  rocks  hot  spring 
action  is  set  up;  and  the  igneous  rocks  themselves  contain 
disseminated  metals  which  they  supply  to  the  circulating 
waters.  As  a  consequence  of  part  or  all  of  these  conditions 
various  kinds  of  ore-deposits  result. 

THE  INTERSECTION  OF  CIRCULATION  CHANNELS 
AS  SEATS  OF  MINERALIZATION. 

What  are  ore-shoots? 

Veins  or  lodes  are  not  usually  equally  rich  throughout. 
Poor  or  barren  spaces  of  lode  separate  ore-bodies  irregular 

*  E.  Haworth,  Bulletin  Geological  Society  America.  Vol.  II,  pp.  231-240. 


196 


GEOLOGY  APPLIED  TO  MINING. 


in  form  or  having  more  or  less  roughly  a  columnar  shape. 
The  latter  are  called  ore-shoots  or  chimneys  (Fig.  47). 
What  is  the  principle  of  intersection  as  regards  ore-deposits? 
A  large  proportion  of  ore-shoots  is  formed  by  the  inter- 
section of  two  water-courses.  This  may  mean  the  inter- 
section of  two  faults,  of  two  joints,  of  a  joint  with  a  fault 
plane,  of  joints  or  faults  with  a  porous  stratum,  etc.  The 


Fig.  47.  Ore-shoot  in  Annie  Lee  mine,  Cripple  Creek,  Colorado,    a,  ore-shoot;  6, 
dike  in  which  shoot  lies.    After  R.  A.  F.  Penfose,  Jr.* 

principle  is  nearly  the  same  throughout;  and  is  explained  in 
Chapter  V.  It  is  at  the  intersection  of  two  circulation  chan- 
nels, whether  now  or  only  formerly  used  by  the  solutions, 
that  one  may  look,  in  almost  any  district,  for  the  richest 
ore-bodies.  In  well-defined  veins,  the  pockets  or  richest 
portions  are  apt  to  lie  at  the  juncture  of  the  main  vein  with 
subordinate  intersecting  fractures  or  veins  (feeders). 

*  16th  Annual  Report  United  States  Geological  Survey,  Part  II. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  197 

ROCK  MOVEMENTS  SUBSEQUENT  TO  ORE-DEPO- 
SITION. 

Do  rock  movements  occur  subsequent  to  ore-deposition? 

In  some  cases  we  find  a  vein  or  ore-deposit  entirely  un- 
affected by  movements  of  the  rocks  after  its  deposition; 
but  generally  there  has  been  some  subsequent  disturbance, 
producing  folding,  faulting,  shearing,  jointing,  fracturing, 
and  fissuring,  which  affect  the  ore-body  in  the  same  way  as 
the  enclosing  rock.  These  movements  may  be  very  slight, 
or  they  may  be  profound. 

How  do  subsequent  movements  diminish  the  value,  of  an  ore- 
body? 

The  bending,  breaking  and  separation  of  the  parts  of  an 
ore-body  or  vein  may  make  it  difficult  to  follow  it  in 
mining;  or  so  expensive  that  the  profit  ^yill  not  pay  for  the 
labor  involved;  or,  sometimes,  practically  impossible. 

Do  movements  subsequent  to  ore-deposition  always  decrease 

the  value  of  a  vein  or  other  ore-body? 

Sometimes  the  disturbances  may  have  an  effect  beneficial 
to  mining.  The  folding  and  faulting  may  so  displace  the  ore- 
body  as  to  make  it  more  accessible  to  mining  operations 
than  it  otherwise  would  be.  Take  the  case  of  a  coal  or  an 
ore-bearing  bed,  for  example,  which  dips  steeply  into  the 
earth.  The  deeper  such  a  bed  is  followed,  the  more 
expensive  and  difficult  becomes  the  mining.  But  if  it 
is  folded  and  faulted  so  that  it  comes  to  the  surface  in  a 
number  of  different  places,  then  it  can  be  easily  worked  at 
each  of  these. 


198  GEOLOGY  APPLIED  TO  MINING. 

DISLOCATIONS  SUBSEQUENT  TO  ORE-DEPOSITION  AS  SEATS 
FOR  LATER  MINERALIZATION. 

Is  the  foregoing  the  only  way  that  movements  subsequent  to  ore- 
deposition  operate  to  increase  the  value  of  a  vein? 

An  ore-body  may  be  traversed  by  joints,  or  fissures, 
which  afford  channels  for  waters  to  circulate,  where  other- 
wise the  openings  have  been  completely  cemented  by  ore 
and  accompanying  gangue.  These  new  openings  may  be 
in  time  partially  or  wholly  cemented  up  with  gangue,  or 
with  ore  and  gangue,  and  frequently  the  waters  will  work 
over  the  old  ore  arid  reprecipitate  it  in  concentrated  form, 
both  in  the  fractures  and  in  the  vein  near  by.  Thus 
these  portions  may  become  the  richest  in  the  vein,  and 
perhaps  the  only  portions  that  it  will  pay  to  work.  Many 
ore-shoots  are  of  this  origin.  Again,  the  new  solutions 
may  bring  fresh  rnetals  from  some  outside  source,  which 
they  may  deposit  in  or  near  the  new  fractures,  either  adding 
them  to  the  earlier  deposited  metals  or  depositing  them 
independently;  and  in  this  way  also  richer  bodies  may  be 
formed  in  the  older  vein. 

In  what  direction  are  the  subsequent  fractures  most  likely  to 
occur  with  respect  to  the  original  veins  or  shoots? 

Movements  in  rocks  are  likely  to  be  very  long  continued, 
though  intermittent;  and  planes  or  zones  of  weakness 
being  once  formed,  renewed  movements  are  likely  to  take 
place  along  them.  The  openings  along  which  ores  are 
formed  are  planes  or  zones  of  weakness,  hence  move- 
ment may  occur  along  them  while  the  first  deposition 


DYNAMIC    AND     STRUCTURAL     GEOLOGY.  199 

is  taking  place,  or  after  it  has  closed.  Even  though 
the  opening  has  been  entirely  cemented  up  by  ore  and 
gangue,  the  regions  of  weakness  will  often  remain  as 
such,  because  the  parallel  parting  planes  of  the  veins  and 
the  encasing  rock  preserve  the  original  slip-surfaces,  and 
because  the  brittle  quartz,  calcite,  etc.,  of  the  vein  may 
be  in  many  cases  more  easily  broken  than  the  tougher 
and  more  yielding  rock.  Therefore,  movements  subse- 
quent to  ore-deposition  are  very  likely  to  fracture  the 
veins  parallel  to  their  course,  the  fractures  either  lying  in 
them  or  alongside  of  them;  and  are  also  likely  to  renew 
the  fractures  whose  intersection  with  the  main  original 
fracture  and  vein  zone  gave  rise  to  ore-shoots.  In  this 
way  later  parallel  bands  in  the  main  vein,  of  different 
character  (both  as  regards  mineral  composition  and  value), 
may  be  produced ;  and  the  old  shoots  may  be  enriched  by  a 
second  deposition. 

RIBBON  STRUCTURE. 

What  is  ribbon  structure? 

Movements  in  veins  subsequent  to  their  formation  may 
produce  a  sheeting  parallel  to  the  walls,  which  may  have 
somewhat  the  aspect  of  original  crustification,  and  may  be 
mistaken  for  it.  This  sort  of  banding  is  called  ribbon 
structure. 

Example:  The  gold-bearing  quartz  veins  of  Nevada  City 
and  Grass  Valley,  California,  typically  show  sheeting  or 
ribbon  structure,  due  to  movement  since  deposition.  True 
original  banding,  or  crustification,  is  also  found  in  these 
veins,  and  often  occurs  in  the  same  specimen  of  rock  as  the 


200  GEOLOGY  APPLIED  TO  MINING. 

ribbon  structure.  Fig.  48  is  a  photograph  of  a  specimen  of 
vein  quartz  containing  gold-bearing  pyrites  from  the 
Providence  mine. 


FAULTED  FAULTS  AND  THEIR  RELATION  TO  ORE-DEPO- 
SITION. 

Are  the  faults  of  one  period  ever  faulted  by  the  faults  of  a  later 
period? 

Where  there  are  a  number  of  faults,  developed  at  different- 
periods,  the  later  movement  may  take  place  along  the  same 
plane  as  the  older  one;  thus  along  an  old  break  the  new 
disturbance  will  continue  the  faulting  and  increase  it. 
Fault  fissures  which  have  become  occupied  by  ore-bearing 
veins  often  experience  such  renewal  of  motion,  and  we  find 
evidence  of  it  in  the  crushed  ore  and  vein  material,  which 
may  subsequently  become  re-cemented  by  new  mineral 
deposition,  and  yet  will  always  show  the  angular  outlines 
due  to  breaking. 

Again,  the  later  faults  may  be  developed  along  planes  not 
parallel  to  the  old  ones,  and  so  cut  and  displace  the  old 
faults  in  precisely  the  same  way  as  the  enclosing  forma- 
tion. Where  ores  have  formed  along  both  the  earlier  and 
later  faults,  one  vein  may  be  found  faulting  another. 

A  specially  complicated  and  likely  case  is  where  faulting 
goes  on  for  a  long  period  slowly,  and  contemporaneously 
with  a  persistent  process  of  ore-deposition.  The  first  ore- 
deposits  may  be  subsequent  to  the  first  folds  and  faults, 
but  they  will  be  disturbed  by  the  later  movements ;  yet  these 
later  faults  may  be  chosen  for  the  seats  of  newer  ore-deposits, 
which  may  again  be  broken  by  still  more  recent  movements, 


DYNAMIC     AND    STRUCTURAL    GEOLOGY.  201 


Fig.  48.  Vein  quartz,  showing  ribbon  structure;  from  Providence  mine,  Grass 
Valley  district,  California.    After  Waldemar  Lindgren.* 

*  17th  Annual  Report  United  States  Geological  Survey,  Part  II,  PL  IX. 


202 


GEOLOGY    APPLIED    TO    MINING. 


and  so  on.  In  such  cases  only  careful  examination  of  the 
phenomena  connected  with  each  separate  ore-deposit  can 
determine  its  age  relative  to  the  various  displacements,  and 
serve  as  a  guide  to  mining  operations. 


Fig.  49.  Faulting  in  Smuggler  and  Molly  Gibson  mines,  Aspen,  Colorado,    o, 

Carboniferous  shales;   6,  porphyry  (intrusive  sheet);   c,  Carboniferous 

limestone;    d,  Carboniferous  dolomite;    e,  Devonian   quartzite 

series;  /,  Silurian  dolomite;   gr,  Cambrian  quartzite; 

h,  Archaean  granite.    After  J.  E,  Spurr. 

Example:  A  good  example  of  successive  faults  acting  in 
different  directions  is  found  in  the  Smuggler  and  Molly 
Gibson  mines,  Aspen,  Colorado.*  Study  of  the  geology 
here  shows  that  first  the  rocks  were  folded  and  acquired 
a  steep  dip  (Fig.  49).  Next  came  the  development  of  the 
Silver  fault,  nearly  parallel  to  the  bedding,  but  of  such  great 


*  J.  E.  Spurr,  Monograph  XXXI,  United  States  Geological  Survey,  pp. 
181-188. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  203 

displacement  as  to  cut  out  the  porphyry  sheet  b  and  the 
limestone  c,  so  that  the  shale  a  was  brought  into  contact 
with  the  dolomite  d  (Fig.  49  B).  Subsequently  came  a 
series  of  east-west  faults  dipping  to  the  south  (such  as  the 
Delia  fault)  which  faulted  the  Silver  fault  together  with  the 
rock  formations  (Fig.  49  C).  Finally  there  came  a  slipping 
on  the  old  plane  of  the  Silver  fault,  which  locally  deviated 
from  that  plane,  and  so  constituted  an  independent  fault 
(Clark  fault) .  The  final  result  is  shown  in  Fig.  49  D.  The 
Silver  fault  was  formed  before  ore-deposition;  the  Delia 
fault  began  to  form  before  ore-deposition,  but  continued 
after  it.  The  Clark  fault  was  formed  after  the  main  ore- 
deposition,  yet  secondary  more  recent  ores  have,  to  a 
slight  extent,  formed  along  it. 

ROCK    MOVEMENTS     ALONG    EARLIER-FORMED 
DIKES. 

Why  do  veins  sometimes  form  along  earlier-formed  dikes? 

Following  up  the  principle  indicated  in  the  fore- 
going pages,  we  may  remark  that  dikes  of  igneous 
rock  are  usually  intruded  along  lines  of  weakness  in 
the  rocks  which  they  cut.  In  the  same  way  as  mentioned 
in  the  case  of  veins,  the  zone  of  weakness,  though  to  a  cer- 
tain extent  cemented  by  the  dike,  is  still  apt  to  remain 
weaker  than  the  rest  of  the  rock.  Any  renewal  of  the 
strains,  therefore,  is  likely  to  produce  a  renewed  fracturing 
along  this  line,  creating  a  new  channel,  which  may  become 
the  passage  of  circulating  waters  and  in  this  way  be  again 
cemented  (this  time  by  water  action),  and  become  a 
mineral-bearing  vein.  The  contact  of  the  dike  with  the 
fractured  country  rock  is  usually  the  weakest  line;  hence, 
later  veins  are  apt  to  occupy  this  position. 


204 


GEOLOGY  APPLIED  TO  MINING. 


Example:  The  Black  Jack-Trade  Dollar  vein,  De  Lamar 
district,  Idaho,  consists  of  a  quartz  and  orthoclase-feldspar 
(valencianite)  gangue  enclosing  rich  silver  and  gold 
minerals  in  small  quantities.  The  lower  part  of  the  vein 
is  situated  at  the  contact  of  granite  with  a  basalt  dike  a 


Fig.  50.  Vein  loilowing  the  course  af  a  pre-existing  dike  Trade  Dollar  vein,  De 

Lamar  district,  Idaho,     a, 'basalt  dike;  6,  granite;  c,  vein  quartz. 

After  W.  Liudgren. 

few  feet  in  thickness.  The  vein  is  separated  from  the  basalt 
by  well-defined  walls  and  gouge,  and  often  shows  comb- 
structure. 

There  is  evidence  that  fracturing  and  faulting  have  taken 
place  at  the  contact  of  the  granite  with  the  basalt  dike, 
involving  a  horizontal  throw  of  125  feet.  Thus  the  old 
fracture  zone,  which  existed  before  the  advent  of  the  dike, 
was  reopened,  and  gave  passage  to  waters  which  deposited 
the  vein*  (Fig.  50). 

*  Waldemar   Lindgren,    20th    Annual    Report    United    States    Geological 
Survey,  Part  III,  pp.  1^4,  165,  etc. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  205 

PART   III. 

PLACERS. 

Why  are  placers  considered  under  the  head  of  dynamic 

geology f 

To  dynamic  geology  belongs  not  only  the  study  of  rock 
movements  beneath  the  crust,  but  also  those  on  the  surface. 
Thus  under  this  head  we  naturally  take  into  consideration 
stratified  ore-deposits,  so  far  as  these  are  chiefly  of  mechan- 
ical origin. 

The  most  important  class  of  stratified  deposits  are  the 
placers.  The  name  placer  is  applied  to  detrital  deposits 
of  metals  or  valuable  minerals,  especially  gold. 

THE  CONCENTRATION  OF  GOLD  IN  PLACERS. 

What  is  the  origin  of  placers? 

Rocks  at  the  surface  are  broken  up  by  erosion  and  find 
their  way,  by  the  power  of  gravity,  aided  by  running  water, 
down  into  the  valleys.  In  the  highest  valleys  only  the 
coarser  fragments  remain,  for  the  streams  carry  away  the 
smaller  ones.  Further  down,  as  the  stream  current  loses 
its  force,  some  of  these  smaller  ones  are  deposited,  and 
only  the  still  finer  material  is  carried  on,  until  nothing  but 
silt  or  mud  is  left.  In  a  gold-bearing  region,  the  veins  are 
broken  up  and  sent  on  their  journej'  in  company  with  the 
detritus  of  the  enclosing  rocks. 


206  GEOLOGY  APPLIED  TO  MINING. 

CONCENTRATION  BY  CHEMICAL  WATER-ACTION. 

How  is  gold  freed  from  associated  baser  metals,  in  the  surface 

outcrops  of  veins? 

Gold,  in  deposits  not  too  close  to  the  surface,  generally 
occurs  in  small  quantities  in  intimate  association  with 
metallic  sulphides,  such  as  pyrite  (iron  sulphide),  "galena 
(lead  sulphide),  arsenopyrite  or  mispickel  (sulph-arsenide 
of  iron),  these  sulphides  being  contained  in  quartz  veins. 
Near  the  surface)  atmospheric  agents  attack  the  veins 
chemically,  and,  if  erosion  is  slow  enough  to  let  these 
agents  exercise  their  full  influence  in  decomposing,  dissolv- 
ing, carrying  away  and  re-depositing  the  various  con- 
stituents, the  result  is  that  the  surface  portion  comes  to 
have  a  different  character  from  the  deeper  part.  The 
sulphides  are  broken  up  and  taken  into  solution;  and  the 
metals  thus  dissolved  are  either  carried  quite  away  or  are 
re-deposited  in  deeper  parts  of  the  vein.  But  gold  is  soluble 
with  much  greater  difficulty  than  most  other  metals ;  hence, 
when  the  sulphides  which  contained  it  are  dissolved,  it  is 
mostly  left  behind,  in  its  native  state,  as  free  gold. 

How  is  gold  purified  and  chemically  concentrated? 

Where  the  surface  rocks  are  decomposed,  the  gold, 
mixed  with  the  debris  produced  by  erosion,  may  then  be 
already  in  the  free  state.  Frequently,  however,  the 
sulphides  outcrop,  or  the  gold  is  imperfectly  separated 
from  other  materials.  Then  in  the  gravels  exactly  the  same 
process  goes  on  as  we  have  described  as  occurring  in  the 
vein  outcrop,  and  (on  account  of  the  great  porosity  of  the 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  207 

gravels,  permitting  atmospheric  waters  to  attack  freely 
every  part),  the  baser  metals  are  carried  away  in  solution, 
and  the  gold  is  left  behind  or  is  dissolved  and  re-precipitated. 
This  is  one  reason  why  so  much  of  the  gold  in  placers,  when 
examined  microscopically,  shows  unscratched  or  even 
crystalline  surfaces,  indicating  chemical  deposition.  Frag- 
mental  pieces  of  gold  may  receive  fresh  coatings  from  solu- 
tions thus  originating;  or  the  solutions  may  deposit  gold 
upon  fragments  of  organic  matter,  or  metallic  sulphides, 
for  these  substances  exert  a  precipitating  effect. 

It  is  even  probable  that  gold  already  deposited  in  the 
native  state  may  be,  to  a  slight  extent,  re-dissolved  and 
re-arranged. 

Some  observers,  seeing  the  evidence  of  this  chemical 
action  in  placers,  have  concluded  that  gold  might  be  intro- 
duced into  the  placers  from  other  localities,  in  solution  in 
surface  waters.  But  it  seems  certain  that  the  ruling  influ- 
ence is  mechanical,  and  that  chemical  influence  is  only 
auxiliary,  producing  further  concentration  and  rearrange- 
ment. 

Why  do  gold  placers  often  occur  near  mines  containing  chiefly 

other  metals,  such  as  silver,  lead,  copper,  etc.? 

The  fact  that  from  all  the  metals  carried  from  a  vein  to 

the  gravels,  only  gold  survives,  the  rest  being  more  or  less 

fully  removed  in  solution,  explains  why  many  rich  silver 

regions,  such  as  the  Comstock  and  Leadville,  were  first 

worked  for  their  gold-bearing  gravels.     The  ores  carry  a 

certain  proportion  of  gold,  and  it  is  this,  freed  more  or  less 

completely  from  the  other  metallic  constituents  of  the 


208          GEOLOGY  APPLIED  TO  MINING. 

ore-deposits,  which  becomes  placer  gold.  Even  ores  con- 
taining only  traces  of  gold  may  thus  give  rise  to  gold- 
bearing  gravels. 

CONCENTRATION  BY  MECHANICAL  WATER-ACTION. 

How  is  gold  mechanically  concentrated  in  placers? 

Particles  of  native  gold  in  gravels,  brought  down  into 
the  valleys  by  mechanical  action,  and  freed  from  other 
metals  and  often  increased  beyond  their  original  size  by 
chemical  action,  are  in  the  valleys  still  further  mechan- 
ically concentrated.  Waters  shift  the  gravels  so  that  the 
heaviest  minerals,  especially  the  gold,  sink  naturally  to  the 
bottom;  and,  where  there  is  not  much  disturbance  by 
running  water,  the  gold  particles  seem  to  be  able  to  work 
downward  through  the  loose  gravel,  probably  during  such 
movement  as  the  deposit  undergoes  from  percolating  waters, 
or  from  alternate  thawing  and  freezing. 

What  is  the  pay-streak  in  gravels? 

The  result  of  the  downward  sifting  of  the  gold  is  that 
where  the  valley-deposits  (sand,  gravel,  etc.)  are  porous, 
by  far  the  greater  quantity  of  gold  will  be  found  at  the  very 
bottom,  in  the  few  inches  overlying  the  bed-rock.  This  is 
called  the  pay-streak. 

How  is  it  that  gold  is  often  found  in  the  bed-rock,  under  the 

gravel? 

The  bed-rock  itself,  (especially  if  it  is  a  shale  or  a  schist, 
and  so  affords  cracks  for  the  gold  to  work  itself  into),  is 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  209 

commonly  rich  for  several  inches  in  depth,  and  is  taken  up 
by  miners  and  worked  with  the  gravels.  This  fact  some- 
times leads  to  the  belief  that  the  gold  was  originally  con- 
tained in  these  rocks ;  but  generally  the  rock  a  little  distance 
below  the  surface  will  be  found  entirely  barren,  disproving 
this  supposition. 

Why  is  the  gold  not  especially  concentrated  in  the  pay-streaks 

of  certain  placers? 

Where  the  gravels  are  not  sufficiently  porous,  the  gold 
cannot  work  itself  down  so  well,  and  as  a  result  it  may 
occur  scattered  throughout  the  whole  deposit,  though  in 
this  case  the  gravel  is  relatively  poorer  per  cubic  yard  than 
the  pay-streak  on  the  bottom  of  porous  gravels,  the  amount 
of  gold  which  is  usually  concentrated  in  the  pay-streak 
being  distributed  throughout  the  mass. 

What  is  a  false  bottom,  and  how  may  it  cause  a  second  pay- 
streak? 

When  there  is,  in  the  deposit,  an  impervious  layer,  such 
as  a  clay  seam,  this  will  arrest  the  downward  working  of  the 
gold,  and  above  it  a  pay-streak  will  be  formed.  A  lava 
bed,  or  a  solid  conglomerate,  may  play  the  same  part. 
Such  an  impervious  layer  is  often  called  the  false  bottom, 
from  having  the  appearance  of  being  the  base  of  the 
gravels.  There  may  be  several  of  these,  with  intervening 
gravel  beds,  one  below  the  other,  each  overlain  by  its  pay- 
streak;  beneath,  the  real  bottom  may  also  have  its  pay- 
streak. 


210  GEOLOGY  APPLIED  TO  MINING. 

Example:  Quartz  creek,  Seward  Peninsula,  Alaska, 
contains  gold-bearing  gravels  on  its  bottom  and  sides 
(Fig.  51).  The  gold  now  (1900)  being  mined  lies  2  or  3 
feet  below  the  surface,  on  what  the  miners  call  the  bed- 
rock, which  is  a  blue  clay,  apparently  intercalated  in  the 
gravels.  This  blue  clay  afforded  a  floor  upon  which  the 
gold  was  concentrated.  The  real  bed-rock  at  this  point 
has  not  been  reached. 


Fig.  51.  False  bottom  of  clay  in  a  gold  placer  deposit.    Cross-section  of  Quartz 
creek,  Kugruk,  Alaska.    Adapted  from  A.  H.  Brooks.* 

What  is  the  origin  and  signification  of  black  sand  and  ruby 
sand  in  gravels? 

By  the  natural  process  of  concentration  other  heavy 
minerals  are  also  collected,  but,  as  none  of  them  are  so  heavy 
as  gold,  they  are  concentrated  to  a  less  degree.  The 
magnetite  which  is  present  in  many  rocks  is  concentrated, 
and  becomes  the  black  sand  or  magnetic  sand  of  miners; 
the  garnets  found  in  many  schists  and  other  metamorphic 
rocks  form  the  ruby  sand,  etc.  So,  when  a  miner  washes  a 
pan  of  gravel  and  gets  a  little  black  sand,  his  experience 
generally  tells  him  that  the  chances  of  gold  are  small,  the 

*  United    States    Geological    Survey,    1901,     'Reconnaissance    in    Nome 
Region,'   etc.,  Fig.  2. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  211 

exact  reason  being  that  the  materials  in  this  gravel  have 
not  been  concentrated.  In  many  regions  the  auriferous 
veins  are  in  rocks  containing  garnet;  and  the  prospector 
rightly  concludes  that  the  presence  of  ruby  sand  is.  a 
favorable  sign.  On  the  other  hand,  the  presence  of  either 
of  these  sands  does  not  necessarily  indicate  even  a  small 
quantity  of  gold. 

EFFECTS  OF  GLACIAL  ACTION. 

What  foundation  is  there  in  the  theory  often  held  by  miners, 
that  glaciers  are  responsible  for  many  placer-deposits? 
Frozen  water,  (snow,  and  especially  ice),  is  a  powerful 
erosive  agent.  It  fills  crevices  in  rocks,  and  by  its  expan- 
sion in  freezing  rends  them  apart;  it  accumulates  in  masses 
on  the  steep  hill  sides  in  high  mountains  as  mountain 
glaciers;  it  moves  down  into  the  valleys  as  valley  glaciers; 
or,  finally,  piling  up  over  mountain  and  valley,  it  forms  a 
great  ice  cap,  or  continental  glacier.  The  slowly  flowing 
ice  grinds  away  the  rock  on  its  bottom  and  sides,  and 
carries  along  on  its  surface  what  slides  down  on  it  from 
cliffs  above.  So  in  glacier  regions  there  is  generally  a 
much  greater  abundance  of  surface  debris  than  in  un- 
glaciated  ones.  It  may  be,  therefore,  that  glaciers  are 
often  effective  in  breaking  up  auriferous  rocks;  but  the 
cases  where  profitable  placers  are  due  entirely  to  glacial 
action  are  probably  few.  This  remark  is  made  because  it 
is  a  favorite  theory  with  miners  "that  the  gold  was  brought 
down  by  glaciers."  Placers  often  occur  in  districts  which 
do  not  have  any  glaciers  and  never  had  any.  It  is  true 


212          GEOLOGY  APPLIED  TO  MINING. 

that  many  regions  now  bare  of  glaciers  were  formerly 
covered  with  them,  such  as  the  great  glaciated  areas  of 
Canada  and  the  North  Eastern  United  States;  but  in  each 
place  we  must  find  the  characteristic  mark  of  glacier 
deposits — unstratified  drift  or  "till,"  ice  scratches,  glacial 
topography,  etc., — before  we  can  allow  this  factor  to  even 
become  possible  in  any  theory  of  placer  formation. 

Besides  the  ground-up,  unstratified  drift  which  is  the 
product  of  the  glacial  mill,  streams  derived  from  the  melting 
of  the  glaciers,  coming  from  their  surface  and  below,  work 
over,  rearrange  and  deposit  the  drift  in  more  or  less  strati- 
fied form.  Such  action  tends  to  classify  the  minerals 
present,  but  the  process  is  generally  incomplete  as  com- 
pared with  that  accomplished  by  streams  in  valleys. 
Hence,  even  stratified  glacial  deposits  are  not  very  favor- 
able for  placers. 

Nevertheless,  ordinary  streams  may  take  up  material 
supplied  by  glacial  action  and  by  "classifying"  it  so  as  to 
shake  the  gold  down  to  the  bottom,  produce  good  placers. 
In  some  cases,  even,  the  material  ground  out  of  auriferous 
rocks  by  glaciers,  and  worked  over  by  glacial  streams, 
may  be  rich  enough  in  concentrated  gold  to  be  valuable. 

Example:  The  Blue  Spur  placers,  in  the  Otago  district, 
New  Zealand,  have  been  described  by  T.  A.  Rickard,* 
who  considers  them  due  to  the  combined  action  of  glacial 
ice  and  glacial  water.  The  deposit  is  probably  as  old  as 
the  Eocene  period,  and  belongs  to  the  class  of  old  placers,  f 
being  above  the  bed  of  the  present  streams. 

*  Transactions  American  Institute  Mining  Engineers,  Vol.  XXI,  pp.  432-436. 
tSeep.  222. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY. 


213 


The  depression  in  which  the  auriferous  gravels  rest  is 
not  that  of  an  ordinary  river  valley,  but  is  rather  an 
irregular  cup-shaped  depression  in  the  schist  (Fig.  52). 
In  the  bottoms  and  sides  of  this  hollow  there  are  also  other 
smaller  hollows  in  the  bed-rock,  such  as  are  not  usually 


// 


^////S:^..v,:- 

'/•&.WX&& 


Fig.  52.  Basin  containing  auriferous  glacial  gravels.    Blue  Spur  placers,  Otago 
district,  New  Zealand,    a,  auriferous  gravels;  6,  schist.  After  T.  A.  Rickard. 

formed  by  running  water  (Fig.  53).     Therefore,  the  basin 
is  believed  to  have  been  scooped  out  by  a  glacier,  to  which 
conclusion  the  occurrence  of  large  boulders  in  the  deposit, 
derived  from  a  locality  25  miles  distant,  lends  support. 
After  scooping  out  the  depressions,  the  glacier  is  sup- 


''/.'    ~,/' 

'  -$tt 

Fig.  53.  Irregular  depressions  filled  with  auriferous  glacial  gravels.    After 
T.  A.  Rickard. 


///'/ 


posed  to  have  retreated,  leaving  the  hollows  to  be  occupied 
by  a  lake.  Materials  ground  out  of  the  auriferous  schists 
by  the  glacier  in  the  new  position  were  carried  into  the 
lake,  where  they  accumulated  in  thick,  coarse,  rudely 
stratified  deposits.  The  gold  gradually  sifted  its  way  to 


214          GEOLOGY  APPLIED  TO  MINING. 

the  lower  portions.  These  deposits  are  now  transformed 
into  hills  by  the  erosion  of  streams,  which  first  drained  the 
lakes,  and  then  cut  down  through  the  lake  sediments. 

VARIOUS  KINDS  OF  STREAM  GOLD  PLACERS. 

What  are  gulch  placers? 

Gulch  placers  are  formed  in  the  highest  narrow  valleys, 
or  gulches,  of  a  river  system.  They  usually  head  in  hills 
or  mountains,  and  the  material  in  their  bottoms,  though 
rudely  stratified,  is  coarse  and  shows  only  slightly  the 
effects  of  wear  and  transportation.  In  the  extreme  upper 
portion  of  the  gulch,  where  it  heads  in  the  bed-rock,  gravel 
is  often  wanting,  but  the  amount  of  it  increases  as  the 
gulch  gains  depth.  The  gulches  are  generally  more  or  less 
V-shaped  in  outline;  hence  the  width  of  the  deposit  is 
slight.  The  gold,  being  near  its  place  of  origin,  is  com- 
paratively coarse.  The  relative  richness  of  various 
gulches,  even  neighboring  ones,  varies  greatly,  according 
to  the  richness  of  the  rocks  through  which  they  cut. 

Example:  Myrtle  creek,  Koyukuk  district,  Alaska,  may 
be  selected  at  random  from  a  host  of  examples.  The 
gravels  here  rarely  exceed  3^  feet  in  thickness,  and  overlie 
mica-schist  and  slate,  which  stand  on  edge.  The  gold  is 
found  principally  on  or  near  bed-rock,  in  the  joints,  fissures, 
and  cleavage  crevices  (Fig.  54). 

What  are  the  characteristics  of  broad  valley  placers? 

The  upper  valleys  or  gulches  unite  further  down  to  form 
larger  and  broader  valleys.  Here  the  stream  flows  in  a 
level  plain  of  gravelly  materials,  which  stretches  back  to 


DYNAMIC     AND    STRUCTURAL    GEOLOGY. 


215 


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216          GEOLOGY  APPLIED  TO  MINING. 

the  valley  sides.  As  the  stream  wears  away  one  bank  and 
builds  up  another,  it  changes  its  position,  and  so,  at  one 
time  it  runs  along  one  side  of  its  valley,  and  at  another 
time  the  opposite  side.  In  this  lateral  swinging  it  works 
over,  classifies  and  smooths  the  gravels  of  its  flood-plain. 
In  auriferous  regions  these  gravels  become  placers. 

The  valley  gravels  are  in  far  greater  quantity  than  the 
gulch  gravels;  and,  since  with  increasing  distance  from  the 
head  of  the  stream  the  gradient  of  the  stream  usually 
decreases,  permitting  increased  deposition,  their  thickness 
is  comparatively  great.  On  account  of  the  more  complete 
work  of  the  swinging  rivers,  the  gold  content  is  apt  to  be 
more  uniform  than  in  the  gulch  placers;  and  since  not  only 
the  rich  but  the  barren  gulches  have  contributed  their 
material,  this  content  is  apt  to  be  considerably  less  than  the 
rich  but  limited  gulch  placers. 

Where  is  the  most  gold  apt  to  be  found  in  valley  gravels? 

Although  valley  placers  attain  often  to  a  considerable 
depth,  the  statements  made  above  concerning  the  working 
downward  of  the  gold  in  gulch  placers,  by  reason  of  its 
gravity,  seem  to  apply  here  also. 

7s  the  gold  of  broad  valley  placers  of  the  same  kind  as  that  in 

the  gulch  placers? 

Naturally,  in  broad  valley  placers  the  gold  is  generally 
of  finer  size  than  in  the  gulches. 

What  are  bar  placers  or  bar  diggings? 
When  a  stream  runs  through  auriferous  gravels  and  by 


DYNAMIC     AND     STRUCTURAL     GEOLOGY. 


217 


undercutting  its  banks  brings  down  and  works  over  large 
quantities  of  these  gravels,  the  gold  undergoes  a  further 


v  ig.  55.  Diagram  of  ideal  river,  showing  accumulation  of  bars.    Crosses  show 
the  most  favorable  spots  for  the  deposition  of  gold.     After  J.  E.  Spurr.* 

concentration  in  the  stream  current.     At  places  where  the 
current  slackens  to  the  right  point,  the  heavy  gold  and  the 

*  18th  Annual  Report  United  States  Geological  Survey,  Part  III,  Fig.  24. 


218  GEOLOGY  APPLIED  TO  MINING. 

coarse  pebbles  are  deposited;  further  on,  the  fine  gold  and 
the  smaller  pebbles  and  sand.  On  such  a  river  "colors" 
(small  flakes)  of  gold  may  be  found  everywhere  in  panning, 
but  the  richest  spot  is  where  the  most  and  the  heaviest  gold 
has  been  deposited,  on  bars.  Fig.  55  shows  the  spots  along 
such  a  stream  where  the  gold  will  best  accumulate.  Bars 
are  always  the  first  point  attacked  by  the  prospector  in  a 
new  country.  They  are  often  very  rich,  but  are  quickly 
worked  out,  and  in  general  do  not  call  for  more  approved 
appliances  than  the  cradle  or  the  long-torn. 

Example:  In  Alaska,  river  bars  were  worked  as  early  as 
1861,  near  the  mouth  of  the  Stikine  river.  Subsequently 
gold  was  found  in  the  bars  of  many  of  the  other  Alaskan 
rivers.  In  1885,  bars  on  the  Stewart  and  Lewes  rivers  were 
worked,  and  soon  afterward  on  Forty  Mile  creek.  Not 
until  1887  did  the  pioneers  advance  from  bar  mining  to 
gulch  diggings,  in  one  of  the  branches  of  Forty  Mile  creek. 
Since  that  time  the  practical  exhaustion  of  the  bars  has 
thrown  all  the  enterprise  into  gulch  mining,  and  even  into 
bench  mining.* 

BEACH    PLACERS. 

What  are  beach  placers? 

When  a  river,  rising  in  a  gold-bearing  region,  reaches 
the  sea  with  a  slow  current,  it  carries  only  fine  mud  or  silt, 
and  the  finest  possible  particles  of  gold.  These  gold  flakes 
almost  float  on  the  water;  they  are  largely  taken  into 
solution  by  the  sea  water,  whence  it  comes  that  this  water 


"*  H.  B.  Goodrich,  18th  Annual  Report  United  States  Geological  Survey, 
Part  III,  pp.  107-125. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  219 

contains  gold  to  the  amount  of  about  11  milligrams  to  the 
ton.*  Part  is  probably  deposited  with  the  settling  mud, 
but  the  amount  is  very  small  and  of  no  direct  com- 
mercial importance. 

But  where  the  rivers  discharge  into  the  ocean  with  a 
strong  current,  they  carry  coarse  rock  fragments  and  gold 
particles  of  considerable  size,  which  are  deposited  on  the 
sea  shore.  The  waves  and  currents  work  this  material 
sidewise  till  it  forms  beaches  extending  along  the  coast. 
The  surf,  continually  moving  that  portion  of  the  material 
which  comes  within  its  reach,  often  effects  a  concentration, 
the  gold  being  accumulated  and  much  of  the  lighter 
material  swept  away.  Shore  ice,  especially  in  northern 
regions,  may  also  be  sometimes  an  important  agency  in 
working  over  the  material.  Thus  beach  placers  are  formed. 

Sometimes  the  shore  waves  undercut  a  gravel  bank  con- 
taining gold,  and  then  concentrate  the  material  in  the 
same  way  as  before.  This  kind  of  beach  placer  differs 
from  that  above  mentioned,  in  that  the  rivers  do  not 
directly  contribute  the  gold  to  the  beach;  yet  they  do  it 
indirectly,  for  the  gravels  undercut  by  the  waves  have 
generally  been  brought  to  this  position  by  rivers — that  is, 
they  are  broad  valley  gravels,  or  they  are  old  sea-shore 
gravels  brought  down  by  former  streams,  and  raised  high 
and  dry  by  an  uplift  of  the  crust. 

Example:  Among  the  most  famous  beach  placers  are 
those  of  Nome,  Alaska,  which  caused  a  stampede  of  many 


*  Luther   Wagoner,    Transactions  American  Institute  Mining   Engineers, 
Vol.  XXXI,  p.  807. 


220          GEOLOGY  APPLIED  TO  MINING. 

thousand  prospectors  in  1900.  These  are  examples 
of  the  type  of  beach  placers  described  in  the  last 
paragraph.  In  this  region  the  rocky  hills  are  bordered 
along  the  sea  by  a  flat  coastal  plain,  composed  of 
auriferous  gravels  brought  down  by  rivers  and  spread  out 
under  the  sea  as  a  marine  shore  deposit,  at  a  time  when  the 
land  was  at  a  lower  level  than  at  present  (Fig.  56).  Sub- 
sequently the  land  was  uplifted,  and  the  marine  gravels 
became  transformed  into  the  present  shore  plain.  The 
strong  waves  of  this  region  cut  back  the  gravel  and  wash  it 
away,  and  concentrate  the  gold,  forming  rich,  but  limited 
deposits.  Therefore  the  gold  in  these  placers  has  been 
successively  concentrated  by  waters  more  than  once. 


Fig.  56.  Diagrammatic  section  of  beach  placers  at  Nome,  Alaska.    After 
Schrader  and  Brooks.* 

Do  beach  placers  extend  seaward  under  the  water? 

Beach  placers,  like  bar  placers,  are  almost  invariably, 
from  their  nature,  shallow  and  hence  short-lived.  They  are 
confined  to  a  narrow  strip  along  the  beach,  for,  even  when 
the  gold  has  been  derived  from  auriferous  gravels  forming 
the  shore,  these  older  gravels  will  be  relatively  much 
poorer,  and  either  will  not  pay  for  working  at  all,  or  must  be 
worked  on  a  larger  scale  at  a  much  smaller  profit  per  ton. 
That  portion  of  the  gravel  seaward  from  the  surf -beat  en 

*  United  States  Geological  Survey,  'Reconnaissance  in  Nome  Region,  1901.' 


DYNAMIC     AND    STRUCTURAL    GEOLOGY. 

zone  will  not  have  undergone  the  concentrating  action  of 
the  surf,  and  will  also  ordinarily  contain  a  very  much 
smaller  proportion  of  gold. 

BENCH    PLACERS. 

What  are  bench  placers  or  bench  diggings? 

A  river  valley  often  shows  along  its  sides  shelves,  terraces, 
or  benches,  part  of  the  old  river  bottom  when  the  stream 
was  at  a  higher  level,  in  which  bottom  it  has  cut  itself  a 
newer  and  deeper  channel  (Fig.  57).  If  the  rock  region  is 
gold-bearing,  and  especially  if  there  is  gold  on  the  bars  and 
in  the  valley  gravels  of  the  present  streams,  then  gold  may 
be  also  looked  for  in  the  gravels  lying  on  these  high  benches. 


NV* 


Fig.  57.  Bench  and  valley  placers.    Section  across  Rye  valley,  Blue  Mountains, 
Oregon.    After  W.  Lindgren. 

Examples:  1.  In  the  Nome  district,  Alaska,  the  sides  of 
the  present  stream  valleys  are  covered  with  gravels  and 
marked  by  terraces  or  benches,  representing  former  levels 
of  the  down-cutting  streams.  Such  benches  occur  at 
elevations  of  from  10  to  100  feet  above  the  present  stream. 
The  gravels  on  them  are  known  to  be  auriferous.* 

2.  Rye  valley,  Blue  Mountains,  Oregon,  is  cut  in  tilted 
Tertiary  lake  beds.  On  its  sides  is  a  series  of  river-cut 
terraces,  on  which  lie  later  (Pleistocene)  gravels,  which  are 

*  Alfred    H.    Brooks,  'Reconnaissance    in    Nome     Region,'    etc.,    United 
States  Geological  Survey,  1901. 


GEOLOGY  APPLIED   TO   MINING. 

gold-bearing  (Fig.  57).  There  are  six  or  eight  of  these 
benches,  of  which  the  largest  is  several  hundred  feet  wide. 
The  gravel  is  coarse  and  well-rolled.  In  the  bed  of  the 
valley  are  still  younger  gravels,  also  gold-bearing. 
The  gold  is  fine,  and  is  richest  at  the  bottom  of 
the  gravel  deposits.* 

OLD    PLACERS. 
What  are  old  placers? 

Sometimes  the  earth's  surface  is  disturbed  by  crustal 
movement.  In  some  cases  the  movement  may  be  a 
general  elevation  or  depression,  while  in  other  cases  a  gentle 
tilting  is  produced,  so  that  one  portion  of  a  given  region 
is  relatively  more  elevated  or  depressed  than  another. 
Again,  the  disturbance  may  be  quite  irregular,  producing  a 
warping  of  the  crust.  After  such  movements  the  rivers 
change  their  velocity  and  often  their  direction,  adjusting 
themselves  to  the  new  slopes  of  the  surface. 

A  certain  river  may  be  running  rapidly  down  a  steeply 
sloping  country,  and,  on  account  of  its  strong  current,  it  is 
steadily  cutting  its  bed  deeper  into  the  rock.  A  gentle 
crustal  movement  occurs,  and  the  lower  portion  of  the  river 
experiences  more  uplift  than  the  upper.  The  gradient  is 
changed;  the  current  becomes  sluggish;  the  stream  ceases 
to  cut  into  the  rock,  and  most  of  the  detritus  which  is 
brought  down  is  not  carried  out  to  sea  as  formerly,  but  is 
smoothed  out  by  the  stream  along  the  valley.  Thus  the 
valley  becomes  more  and  more  deeply  covered  with 

*  W.  Lindgren,    22d     Annual  Report  United   States  Geological    Survey, 
Part  II,  p.  788. 


DYNAMIC     AND    STRUCTURAL    GEOLOGY. 


223 


gravels,  and  hence  more  and  more  shallow,  and  it  may  end 
by  being  entirely  filled  up. 

On  the  other  hand,  take  a  region  of  sluggish  streams 
which  have  filled  up  their  valleys  with  gravels,  and  think  of 
the  region  being-  slightly  tilted  so  that  the  streams  begin  to 
run  rapidly.  If  the  tilt  is  in  the  direction  of  the  old  streams, 
the  new  ones  will  have  much  of  the  former  direction,  but,  if 
it  is  in  other  directions,  the  new  rivers  may  flow  at  right 
angles  to  the  old,  or  even  in  the  opposite  direction,  for  a 
country  whose  rivers  have  filled  up  their  valleys  will  be 
nearly  flat  and  will  permit  the  streams  to  change  their  beds 


Fig.  58.  Generalized  section  of  an  old  placer,  with  technical  terms  as  used  in 

California,    a,  volcanic  cap;  6,  upper  lead;  c,  bench  gravel;  d,  channel 

gravel;  e,  bed-rock;  /,  rim.    Adapted  from  R.  E.  Browne.* 

easily.  In  any  case,  new  channels  are  cut.  When  this  is 
accomplished,  the  old  river  gravels  will  be  left  between  the 
new  valleys,  and,  in  proportion  as  the  latter  are  deeply 
cut,  the  former  becomes  relatively  high  above  them. 
These  old  gravels  may  have  been  placers,  and,  when  thus 

*  Report  California  State  Mineralogist,  1890,  p.  437. 


224          GEOLOGY  APPLIED  TO  MINING. 

left  on  or  under  the  hills  above  the  present  valleys,  they 
may  be  called  old  placers  (Fig.  58). 

Examples:  1.  The  auriferous  gravels  of  the  Sierra 
Nevada,  in  California,  are  classic  examples  of  old  placer 
gravels.  They  represent  Tertiary  streams  which  had 
entirely  different  valleys  from  those  of  the  present  day. 
These  old  gravels  have  been  sufficiently  explored,  by  drift 
mining  and  in  other  ways,  to  show  in  general  how  the 
Tertiary  valleys  lay  and  in  what  directions  the  streams 
ran;  moreover,  the  general  surface  of  the  Tertiary  land, 
between  the  valleys,  can  be  ascertained.  The  accom- 
panying map  by  W.  Lindgren*  (Fig.  59)  shows  the  topog- 
raphy of  this  period,  with  the  present  topography  beneath 
in  fainter  lines.  From  this  map  it  may  be  seen  that  while 
the  features  of  the  Tertiary  (Neocene)  topography  showed 
prominent  relief,  the  surface  was  much  less  cut  by  deep 
ravines  than  at  present.  In  general  the  present  streams 
run  at  right  angles  to  the  former  ones. 

After  the  old  auriferous  gravels  were  laid  down,  great 
flows  of  volcanic  rock  filled  up  the  valleys,  and,  as  the 
tilting  took  place  at  about  the  same  time,  the  new  streams 
followed  entirely  independent  sources.  They  have  now 
cut  down  their  beds  so  as  to  expose  the  Tertiary  gravels  at 
many  points. 

2.  In  the  Otago  gold  district,  New  Zealand,  described  by 
T.  A.  Rickardf,  the  rivers  flow  rapidly,  and  in  large  part 
run  through  narrow  gorges  which  they  have  recently  cut. 
They  have  excavated  their  courses  down  below  the  level  of 
their  ancient  valleys,  which  were  mostly  broad  and  filled 


*  17th  Annual  Report  United  States  Geological  Survey,  Part  II,  PI.  II. 
t  Transactions  American  Institute  Mining  Engineers,  Vol.  XXI,  p.  411. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY. 


225 


22G          GEOLOGY  APPLIED  TO  MINING. 

with  alluvium.  The  gravel  in  these  old  valleys  (of  lower 
Miocene  and  Eocene  age)  forms  the  placer  diggings  of  the 
present  day  (Fig.  60). 

Old  placers,  being  generally  old  broad  valley  placers,  are 
commonly  very  thick,  and  contain  the  most  gold  in  the 
lower  part,  the  main  pay-streak  being  as  a  rule  just  above 


OLD 

LAMMERLAW   CREEK 


NEW 
LAMMERLAW  CREEK 


PLAN  SECTION 

Fig.  60.  Old  auriferous  gravels  (Miocene),  Otago  district,  New  Zealand. 
After  T.  A.  Rickard. 


bed-rock.  On  account  of  their  position,  they  are  well 
adapted  for  hydraulic  mining,  and  on  account  of  their 
usually  easy  drainage  by  tunnels,  for  drift  mining. 

FOSSIL    PLACERS. 
What  is  meant  by  the  term  fossil  placers? 

The  old  placers  proper  are  usually  of  Tertiary  age;  they 
are  plainly  river  deposits  belonging  to  an  age  just  preceding 
the  present,  and,  save  for  thin  lava  beds  or  barren  gravel 
deposits,  they  are  not  covered  by  younger  beds.  But 
placer  gravels  may  be  of  any  age.  They  may  be  hardened 
into  rocks  (conglomerates),  and  be  folded  and  faulted  so  as 
to  lose  all  evidence  of  their  original  relation  to  any  stream. 


DYNAMIC     AND    STRUCTURAL     GEOLOGY.  227 

They  may  be  deeply  buried  by  the  accumulation  of  later 
beds;  and,  when  again  exposed  by  erosion,  they  may  out- 
crop either  in  the  mountains  or  in  the  valleys.  But  they 
will  often  still  retain  the  gold  that  was  in  them  origi- 
nally, and  may  be  profitable  for  mining;  or,  when  they 
are  attacked  by  erosion,  the  gold  will  accumulate  in 
stream  bottoms  to  form  a  new  generation  of  placers. 

The  number  of  instances  where  fossil  placers  permit  of 
productive  mining  is  not  so  large  as  might  be  expected ;  but 
it  is  probable  that,  in  many  cases  not  yet  recognized, 
modern  placers  derive  their  gold  from  old  conglomerates 
which  are  of  this  nature.  Solid  rocks  must  contain  many 
times  more  gold  than  loose  gravels  to  be  equally  profitable. 

Example:  On  Pole  creek,  a  branch  of  Cherry  creek, 
Madison  county,  Montana,  a  thick  conglomerate  (maximum 
thickness  500  feet)  lies  unconformably  below  Cambrian 
beds,  and  above  Archaean  gneisses  and  schists.  This  con- 
glomerate seems  to  be  auriferous  throughout  its  extent,  and 
the  gold  in  it  has  been  explained  as  the  result  of  mechanical 
concentration  on  the  shores  of  the  pre-Cambrian  ocean. 
It  is  thus  a  fossil  beach  placer  of  pre-Camhrian  age.* 

RE-CONCENTRATED    PLACERS.f 

Any  one  of  the  classes  of  placers  above  mentioned  may 
have  derived  its  material  wholly  or  partly  from  older 
placers.  Thus  the  gold  may  have  to  pass  through  several 


*  A.  N.  Winchell,  Transactions  American  Institute  Mining  Engineers,  Feb.- 
May,  1902. 

t  This  term  was  first  used  by  Alfred  H.  Brooks.  'Reconnaissance  in  the 
Cape  Nome  Region.'  Alaska,  United  States  Geological  Survey,  1901,  p.  149. 


228         -  GEOLOGY  APPLIED  TO  MINING. 

successive  concentrations,  at  different  times,  before  it  can 
render  a  placer  profitable.  A  gulch  placer  may  represent 
the  re-concentrated  remains  of  a  bench  placer,  and  a  bar 
p'acer  may  be  a  re-concentration  of  the  gulch  placer. 
Similarly  a  fossil  placer  may  be  attacked  by  erosion  and 
its  gold  concentrated  anew.  Numerous  other  combina- 
tions have  commonly  occurred. 

Example:  On  the  river  Galliko.  in  Macedonia,  in  the 
district  of  Kilkitch,  are  placers  worked  at  least  as  early  as 
the  time  of  Philip  of  Macedon,  the  father  of  Alexander 
the  Great.  According  to  modern  ideas,  they  are  very 
low  grade,  and  are  washed  only  by  a  few  peasants.  The 
orig'nal  source  of  the  gold  is  in  veins  which  contain  silver- 
bearing  galena,  carrying  small  quantities  (usually  only 
traces)  of  gold.  These  veins  have  been  extensively  eroded, 
the  silver  and  lead  have  been  dissolved  and  carried  away, 
and  the  gold  has  been  left  in  fine  particles  in  the  gravels. 

During  the  late  Tertiary  period  a  great  thickness  of  such 
gravels  accumulated  in  the  broad  valleys  of  the  Tertiary 
rivers.  Although  the  gold  in  these  gravels  is  very  scarce 
it  is  nevertheless  everywhere  presentj  and  the  deposits  may 
be  considered  as  broad  valley  placers. 

In  these  old  gravels  certain  definite  ancient  stream  beds, 
usually  marked  by  coarser  gravel,  can  be  distinguished,  and 
in  such  channels  the  gold  is  much  more  abundant  than 
elsewhere.  These  are  probably  accumulated  bar  or 
stream  placers  formed  in  streams  which  ran  through  the 
broad  valley  deposits,  and  derived  their  gold  from  them. 
This  is  the  second  stage  in  the  concentration. 

Since  the  formation  of  the  gravels  the  land  has  been 
slightly  uplifted,  and  streams  have  cut  down  through  them. 
The  placers  worked  to-day  are  the  sands  in  the  present 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  229 

stream-beds.  A  small  part  of  the  gold  in  these  placers  is 
derived  directly  from  the  veins,  but  most  of  it  comes  from 
the  older  gravels.  Where  the  stream  cuts  one  of  the  above- 
mentioned  ancient  stream  channel  the  deposit  is  rela- 
tively rich.  This  is  the  third  stage  of  concentration,  and 
only  at  this  point  is  the  gold  sufficient  in  quality  to  be 
worked. 


PLACERS  OTHER  THAN  GOLD  PLACERS. 

What  are  the  characteristics  of  platinum  placers? 

The  geology  of  platinum  placers  is  like  that  of  gold 
placers.  By  its  resistance  to  atmospheric  agents  of 
decomposition,  platinum,  like  gold,  retains  its  integrity 
while  other  minerals  are  decomposed  and  dissolved;  and 
by  reason  of  its  great  weight,  it  is  left  behind  in  streams 
where  lighter  material  is  carried  away.  Thus  a  natural 
concentration  is  effected.  Platinum  is  often  found  with 
gold  in  placers,  a  circumstance  to  be  explained  by  their 
common  resistance  to  disintegration,  and  their  common 
great  specific  gravity. 

Example:  The  platinum  placers  of  the  Ural  Mountains, 
in  Russia,  have  been  the  most  productive  in  the  world. 
Along  the  Tura  river  and  its  tributaries  the  placer  gravels 
have  a  width  of  from  400  feet  to  half  a  mile.  The  gravel 
varies  from  8  to  24  feet  in  thickness,  and  is  overlain  with 
peat  about  4  feet  thick.  The  richest  gravels  are  those 
directly  above  the  bed-rock,  not  exceeding  4  feet  in  thick- 
ness. The  pebbles  are  nearly  angular,  with  frequent  large 
boulders,  and  are  entirely  due  to  river  action.  Gold  and 
platinum  occur  together  in  the  placers  (Fig.  61). 


230 


GEOLOGY   APPLIED   TO   MINING. 


Fig.  61.  Platinum  placers.    Plan  of  a  portion  of  the  river  Iss,  northern  Ural 
Mountains,  Russia.    After  A.  Zaitseff.* 

What  are  the  characteristics  of  tin  placers? 

A  large  proportion  of  the  world's  supply  of  tin  occurs  in 
placers,  usually  called  stream-works.  The  metal  is  in  the 
form  of  the  oxide,  cassiterite,  a  heavy  black  mineral  that 
does  not  have  a  metallic  appearance,  but  has  a  dull  stony 

*  'Pie  PJatinlagerstatten  am  Ural.' 


DYNAMIC     AND     STRUCTURAL     GEOLOGY. 


231 


look.  It  is,  however,  nearly  as  heavy  as  metallic  iron, 
and  in  gravels  tends  downward.  Besides  this,  it  is  with 
great  difficulty  attacked  by  decomposition.  Tin  placers 
are  made  up  from  the  debris  of  tin-veins  in  the  solid  rock, 
and  wherever  one  of  these  deposits  is  found  the  other  may 
be  looked  for. 

Example:  In  the  district  of  Siak,  Sumatra,  alluvial  tin, 
often  in  workable  quantities,  is  found  in  the  gravels  in  the 


HUMUS- 


YELLOW  CLAY 3  TO  3.5    FT 

ANGULAR  QUARTZ  AND  CARBONIZED  WOOD      0.7    FT. 

FINE  GRAY  CLAY 0.15  FT. 

PAY  GRAVEL 0.20  FT. 


TOUGH  GREV  CLAY 10  TO  11.00  rr. 


r=l 


YELLOW  DECOMPOSED  ROCK 8  TO  4,00  FT. 


r  IMPURE  SANDSTONE,  BED-ROCK 


Fig.  62.  Section  of  tin  placer,  Kotta  Ranah,  Siak  district,  Sumatra. 
After  C.  M.  Rolker. 

stream  valleys.  The  bed-rock  consists  of  impure  sand- 
stones and  quartzites,  with  some  silicious  granite  con- 
tain'ng  muscovite  and  tourmaline — minerals  frequently 
associated  with  tinstone.  The  stream  channels  are  covered 


232  GEOLOGY  APPLIED  TO  MINING. 

by  a  shallow  gravel  deposit,  growing  shallower  toward  the 
heads  of  the  streams,  till  bed-rock  is  reached.  The  tin- 
bearing  stratum  consists  of  angular  quartz  gravel  and 
tinstone.  The  pay-streak  is  only  a  few  inches  thick,  and 
is  overlain  by  a  bed  of  quartz  gravel  containing  little  tin- 
stone; and  this  by  sandy  clay  and  surface  vegetable 
accumulation.  It  is  underlain  by  a  tough  gray  clay, 
which,  as  in  the  case  of  gold  placers,  has  formed  a  false 
bottom  or  false  bed-rock,  by  its  imperviousness.  It  is 
probable  that  the  tin  is  derived  from  tin-bearing  quartz 
veins  in  the  underlying  sandstone*  (Fig.  62). 

What  is  th$  origin  of  diamond  placers? 

In  many  parts  of  the  world  diamonds  accumulate  in  the 
stream  sands  and  are  recovered  by  washing.  The  sands 
are  made  up  of  the  debris  of  the  diamond-bearing  rock. 
In  this  case  the  diamond  owes  its  preservation  not  to  its 
specific  gravity,  which  is  not  great,  but  rather  to  its  hard- 
ness and  freedom  from  decomposition,  which  preserve  it 
when  other  minerals  disintegrate  and  are  washed  away. 

What  other  minerals  are  concentrated  in  placers? 

Among  others,  monazite,  a  phosphate  of  the  cerium  metals 
(usually  cerium,  lanthanum,  didymium)  which  has  of  late 
years  been  sought  after  for  use  in  the  making  of  mantles  of 
incandescent  gas  burners,  has  thus  been  obtained.  In 
rocks  it  occurs  scattered  in  small  crystals  and  is  not  com- 
mercially available;  but  it  is  capable  of  being  separated 
from  the  gravels.  In  this  case  the  mineral  has  a  consid- 


*  C.  M.  Rolker,   Transactions  American  Institute  Mining  Engineers,  Vol. 
XX,  pp.  50-84. 


DYNAMIC     AND     STRUCTURAL     GEOLOGY.  233 

erable  specific  gravity — equal  to  that  of  magnetic  iron 
ore — and  this  operates  chiefly  in  the  concentration  in  the 
gravels.  Like  other  placer  minerals,  it  is  not  easily 
attacked  by  atmospheric  agents  of  decomposition. 

RESIDUAL    DEPOSITS 

What  are  residual  or  rooted  deposits? 

Residual  or  rooted  deposits  form  in  weathered  and 
softened  portions  of  the  solid  veins  (root  deposits).  In 
some  countries  where  erosion  has  not  been  very  active  in 
sweeping  away  the  surface  accumulations,  decomposed 
rock  in  place  extends  to  a  great  depth.  In  this  loose 
surface  material  the  gold,  changed  to  its  native  state  by  the 
atmospheric  agents  above  mentioned,  shows  a  tendency 
to  work  downward.  The  concentration  is  aided  effectu- 
ally by  the  winds,  which  blow  away  the  finer  and  the 
looser  material  and  leave  the  heavier  particles.  The  rain 
serves  the  same  purpose  and  removes  material  both 
mechanically  and  in '  solution.  Such  deposits  are  not 
necessarily  in  stream  valleys,  but  are  found  in  flat  countries 
or  on  the  side  of  slopes;  the  parent  or  root  deposit  in  the 
solid  rock  is  never  far  distant,  and  may  directly  underlie 
the  residual  deposit.  On  account  of  the  mechanical  and 
chemical  concentration  which  it  has  undergone,  the  residual 
deposit  is  very  often  richer  than  the  vein  beneath. 

Example:  The  placers  of  Kotchkar,  in  the  Ural  Moun- 
tains, are  mostly  found  immediately  under  the  sod  and  in 
intimate  relation  with  the  outcrops  of  the  auriferous  veins. 


234  GEOLOGY  APPLIED  TO  MINING. 

Their  course  does  not  depend  on  the  surface  configuration, 
but  coincides  with  that  of  the  veins,  into  which  they  grad- 
ually pass,  at  a  depth  of  from  20  to  75  feet.* 

Are  rooted  or  residual  deposits,  other  than  those  of  gold, 

known  to  occur? 

Iron  ores  are  sometimes  formed  in  this  way,  some  bodies 
of  phosphate  of  lime,  and  other  mineral  deposits. 


*  N.  Wyssotzky.       'Les  Mines  d'Or  de  Kotchkar,'    Memoires  du  comit^ 
Geologique,  St.  Petersburg,  Vol.  XIII,  No.  3,  p.  211. 


CHAPTER   V. 

THE  STUDY  OF  CHEMICAL  GEOLOGY  AS  APPLIED 
TO  MINING. 



THE   STUDY   OF   ORE -CONCENTRATION. 

The  study  of  the  chemical  laws  by  which  many  geologic 
changes  take  place  is  of  important  economic  value.  The 
knowledge  of  these  laws  is  beginning  to  throw  greater 
light  upon  the  nature,  probable  extent  and  probable  value 
of  a  given  ore-deposit,  so  that  features  can  frequently  be 
predicted,  the  foreknowledge  of  which  is  of  immense  value 
to  the  miner. 

As  already  explained,  the  rock-forming  elements  are 
constantly  in  a  state  of  movement  and  change.  The 
waters  which  traverse  the  rocks  of  the  crust  are  perhaps  the 
most  powerful  agents  accomplishing  the  change;  they  are 
continually  dissolving,  transporting  and  re-depositing.  In 
the  process  of  solution  they  select  certain  elements  in 
preference  to  the  rest,  and  in  the  process  of  precipitation 
they  concentrate  them.  Thus,  concentrations  of  most  of 
the  elements,  to  a  greater  or  less  degree,  are  brought  about 
in  various  places.  In  the  case  of  the  valuable  rarer  miner- 
als, we  call  such  concentrations  ore-bodies. 


236          GEOLOGY  APPLIED  TO  MINING. 

The  important  thing,  then,  is  to  inquire  how  these 
metals  migrate  and  segregate — what  is  the  agency  and  what 
are  the  qualities  by  virtue  of  which  this  is  accomplished. 

What  is  the  work  of  mechanical  and  chemical  processes, 

respectively,  in  ore-deposition? 

The  mechanical  agencies  have  been  discussed,  in  the 
first  chapter,  in  the  chapter  on  stratified  rocks,  and  in  the 
chapter  on  dynamic  geology.  They  are  almost  wholly 
at  the  earth's  surface.  Chief  of  them  is  water,  or  rather 
gravity  aided  by  water;  to  a  very  slight  extent,  wind  also. 
Particles  of  many  kinds  of  minerals  are  shaken  up  and 
"classified,"*  in  moving  water,  flowing  downward  in 
streams  to  the  sea  under  the  influence  of  the  earth's  gravity; 
and  along  sea  margins,  in  waters  moved  by  tides  or  set  in 
motion  by  the  winds. 

The  chemical  agencies  are  extremely  active  by  sea  and 
land,  on  the  earth's  surface  and  under  it  They  are  very 
largely  responsible,  by  their  disintegrating  action,  for  the 
breaking  up  of  the  surface  rock. into  small  pieces,  which  are 
sorted  over  by  mechanical  agencies  so  as  to  form 
detrital  ore-deposits.  Moreover,  by  themselves  alone  they 
accomplish  the  chief  work  of  ore-concentration. 

What  is  the  principal  agent  in  the  chemical  concentration  of 

ores? 

Many  substances  are  chemically  active  in  the  migration 
of  materials  to  form  concentrations,  but  the  chief  vehicle 

*  This  word  is  used  in  its  technical  meaning  as  regards  artificial  ore-concen- 
tration. 


CHEMICAL    GEOLOGY.  237 

for  all  these  is  undoubtedly  water.  Being  dissolved  in 
water,  these  substances  acquire  the  property  of  motion 
and  so  can  exercise  their  concentrating  forces,  which 
otherwise  must  have  been  powerless.  Outside  of  solution 
in  water,  the  chief  method  by  which  these  substances  can 
acquire  the  power  of  motion  is  by  volatilization— changing 
into  the  gaseous  state,  or  by  passing  into  solution  in  gases. 
Of  all  the  gases  which  thus  play  a  part,  water-vapor  or 
steam,  is,  again,  probably  by  far  the  most  important. 

THE    SHALLOW   UNDERGROUND    WATERS. 

What  is  the  source  of  the  ground-water? 

Most  of  the  water  that  falls  on  the  earth  as  rain  or  snow 
sinks  into  the  rocks;  the  rest  is  evaporated  or  flows  into 
lakes  or  into  the  ocean.  Thus  there  is  a  great  body  of 
subterranean  water. 

What  is  the  level  of  ground-water? 

If  we  sink  a  well,  at  first  we  encounter  no  water,  but  at 
some  depth  the  water  oozes  into  it  from  every  crevice  in 
the  soil  or  rock,  and  after  gathering,  it  stands  at  a  certain 
level.  This  has  been  called  the  level  of  ground- water;  it  is 
near  the  surface  in  wet  regions,  and  deeper  in  drier  regions. 
After  a  rain  the  water  level  rises  and  is  often  at  the  surf  ace. 
When  there  is  little  rain  it  sinks,  so  that  many  wells  go  dry. 

The  surface  of  ground- water  is  not  horizontal  .in  a  hilly 
country,  but  follows  in  a  general  way  the  topographic 
surface,  though  it  is  less  accentuated,  being  further  from 
the  surface  on  hilltops  and  often  practically  corresponding 


238  GEOLOGY  APPLIED  TO  MINING. 

with  it  in  the  valleys.  Yet  a  well  sunk  on  the  top  of  a  hill 
will  generally  find  water  long  before  reaching  the  level  of 
the  valley. 

Is  the  ground-water  everywhere  present  in  the  rocks,  and  to 

what  depths  does  it  extend? 

The  distinction  of  the  rock  above  and  below  the  ground- 
water  surface  has  been  insisted  upon  by  the  latest  and  best 
writers  on  ore-deposits.  They  have  even  carried  the 
theory  so  far  as  to  give  to  the  subject  an  ideal  aspect.  It 
has  been  conceived  that  below  the  ground-water  level  all 
the  openings  in  the  rock,  of  whatever  kind,,  are  saturated 
with  water,  whence  the  phrase  "sea  of  ground- water"  has 
originated,  which  sea  is  conceived  to  extend  to  a  great 
depth — several  miles.  But  it  is  possible  that  this  concep- 
tion is  a  little  too  much  generalized.  In  many  deep  mines, 
the  water  is  nearly  all  encountered  in  the  upper  levels,  and 
the  deeper  portions  are  often  dry,  even  dusty  This 
suggests  that  the  standing  body  of  ground-water  does  not 
in  general  extend  to  a  depth  of  10,000  feet,  or  anything 
like  it,  but  that  its  limit  is  something  like  2,000  feet,  and  in 
many  regions  500  feet.*  Some  writers  are  of  the  opinion 
that  the  amount  of  water  which  descends  into  the  earth 
below  2,000  feet  is  slight,  and  that  it  only  attains  great 
depths  by  comparatively  large  fissures,  which  are  excep- 
tional. Numerous  cases  of  deep,  perfectly  dry  mines, 
which  find  water  (frequently  warm)  on  tapping  some 
fissure,  support  these  ideas. 

*  J.   F.   Kemp,    Transactions  American  Institute  Mining   Engineers,  Vol. 
XXXI,  p.  187,  etc. 


CHEMICAL   GEOLOGY.  239 

The  penetration  of  ground-water  into  rocks  is  exceed- 
ingly irregular.  In  one  place  it  sinks  deeply  and  freely  by 
means  of  rock-openings  and  in  another  it  is  almost  com- 
pletely shut  out  by  dense  and  impervious  rocks.  It  is 
probable  that  the  universal  presence  of  ground-water  is 
characteristic  chiefly  of  a  comparatively  shallow  surface  belt, 
below  which  the  water  which  has  not  been  again  drawn  off 
at  the  surface,  at  a  lower  level,  or  has  not  been  used  up  in 
hydration  processes,  is  concentrated  into  the  larger  fissures. 

Example:  Prof.  Vogt*  has  written  of  several  deep  mines 
in  Norway  in  which  the  lowest  pump-station  is  only  about 
250  meters  from  the  surface.  In  one  of  them,  the  water 
for  use  in  drilling  below  that  level  has  to  be  carried  down. 

THE  WORK  OF  UNDERGROUND  WATERS  IN 
DISSOLVING   ROCKS. 

How  can  underground  waters  dissolve  rock-minerals  .on  an 

extensive  scale? 

Deep-seated  waters  are  usually  ascending,  and  not 
infrequently  heated.  Surface  water,  in  a  cold  state,  is 
capable  of  dissolving  many  materials  from  the  rocks 
through  which  it  passes.  Heating  the  water  augments  its 
activity  in  dissolving  materials,  and  permits  it  to  take  into 
solution  a  greater  quantity  of  foreign  matter;  pressure  in 
general  has  a  similar  effect.  Moreover,  some  of  the  mate- 
rials taken  into  solution  by  water,  or  mechanically  held  in 
it  in  small  separate  particles,  increase  its  solvent  power  to  a 

*  Transactions  American  Institute  Mining  Engineers,  Vol.  XXXI,  p.  165. 


240  GEOLOGY  APPLIED  TO  MINING. 

great  degree.  The  most  important  of  these  substances  is 
probably  carbon  dioxide  (or  carbonic  acid),  which  is  espe- 
cially powerful  in  attacking,  and  taking  into  solution, 
materials  but  slightly  soluble  in  pure  water,  such  as  quartz 
and  many  silicates,  as  well  as  metallic  sulphides.  Hydro- 
gen sulphide  is  also  abundant  in  many  ascending  waters,  as 
in  sulphur  springs.  By  uniting  with  a  little  oxygen,  the 
.gas  changes  into  sulphuric  acid,  which  transforms  many 
difficultly  soluble  salts  into  easily  soluble  sulphates.  Alka- 
line solutions  and  especially  alkaline  sulphides  in  solution 
assist  the  solution  of  gold  and  certain  metallic  sulphides, 
such  as  those  of  silver  and  copper.  Certain  substances  in 
solution  are  especially  favorable  to  the  solution  of  certain 
other  salts.  For  example,  gold  is  soluble  in  ferric  sulphate, 
in  alkaline  iodides,  in  sodic  and  potassic  chloride,*  in  sodic 
carbonate,f  sodic  sulphide,!  sodic  sulphydrate,  etc.  Iodine 
or  chlorine  in  solution  unites  with  certain  metals,  such  as 
gold,  and  makes  an  easily  soluble  iodide  or  chloride.  From 
common  salt  (sodium  chloride),  in  saline  waters,  hydro- 
chloric acid  may  be  formed  by  ferric  sulphate  or  sulphuric 
acid;  and  manganese  oxides,  operating  upon  this  hydro- 
chloric acid,  would  produce  free  chlorine,  which  might  then 
act  on  metals  as  described  above. 

In  fact,  the  processes  of  solution  in  waters  are  almost 
endless  and  many  of  them  are  very  complex.  The  essential 
thing  to  understand  is  that  in  all  waters  these  processes  are 


*  Egleston,  Transactions  American  Institute  Mining  Engineers,  Vol.  VIII, 
p.  455. 

t  Doelter,    'Chemische  Mineralogie,'   Leipzig,  1890. 

J  Becker,  Monograph  United  States  Geological  Survey,  Vol.  XII,  p.  433. 


CHEMICAL   GEOLOGY.  241 

active  and  thus  that  all  waters  are  capable  of  taking  most 
mineral  matters  into  solution;  and  that  deep-seated  waters, 
on  account  of  the  generally  greater  temperature  and 
pressure,  as  well  as  on  account  of  the  intimate  way  in 
which  they  pass  through  rocks,  and  the  length  of  time  that 
they  spend  in  passage,  are  more  powerful  solvents  than 
surface  waters.  Therefore,  given  metals  to  dissolve,  such 
as  are  found  in  desseminated  form  in  most  igneous  rocks 
and  in  some  sedimentary  ones,  and  given  waters  circulating 
through  these  rocks,  we  may  be  sure  that  the  underground 
waters  will  become  charged  to  a  certain  extent  with  the 
metals,  as  well  as  with  most  of  the  other  mineral  substances 
which  they  encounter  in  transit. 

Do  we  know,  as  a  matter  of  fact,  that  underground  waters  do 

carry  mineral  matter  in  solution  ? 

It  is  not  only  by  theory  that  we  come  to  this  conclusion. 
In  many  waters,  it  is  true,  the  amounts  of  some  of  the 
rarer  elements  (such,  for  example,  as  the  precious  metals) 
held  in  solution,  are  so  small  that  we  are  unable  to  detect 
them  chemically;  but  in  other  waters  analyses  prove  their 
presence.  Rain  water,  having  been  evaporated,  has 
become  purified  by  a  natural  process  of  distillation;  but 
ground-waters  always  contain  small  amounts  of  the 
elements  of  the  rocks  or  soils  they  have  traversed.  Many 
waters  contain  iron,  which  they  deposit  as  soon  as  they 
become  exposed  to  the  oxidizing  effect  of  the  atmosphere; 
thus,  in  many  swamps  and  stagnant  pools  we  find  a  red 
deposit  of  hydra  ted  iron  oxide.  In  limestone  regions,  the 
waters  contain  a  considerable  amount  of  lime.  Cold 


242          GEOLOGY  APPLIED  TO  MINING. 

springs  may  contain  many  different  salts  in  solution,  such 
as  various  earthy  and  alkaline  carbonates,  sulphates,  and 
chlorides,  silica,  etc.  The  hot  springs  which  issue  at  the 
surface,  or  are  tapped  by  mining  explorations  underground, 
are  still  more  highly  charged  with  these  substances. 

What  rare  mineral  elements  and  metals  are  found  in  under- 
ground waters? 

A  familiar  example  is  the  natural  lithia  water,  which 
occurs  in  springs  and  contains  a  considerable  quantity  of 
the  salts  of  the  element  lithium,  not  abundant  in  nature. 
This  is  used  for  medicinal  purposes.  Of  more  immediate 
interest  to  the  subject  of  ore-deposition  is  the  established 
presence  of  the  salts  of  arsenic,  antimony,  zinc,  lead,  tin, 
etc.,  even  of  gold,  n  many  mineral  springs. 

Examples:  1.  The  deep  waters  from  the  2,000  foot  level 
of  the  Geyser  mine,  Silver  Cliff,  Colorado,  were  found  by 
W.  F.  Hillebrand  to  contain  silica,  alumina,  iron,  manga- 
nese, lime,  strontium,  magnesium,  potassium,  sodium 
and  lithium  compounds,  together  with  carbonates  of  lead, 
copper,  and  zinc. 

2.  The  springs  at  Rippoldsau  and  Kissingen,  southwest 
Germany,  have  been  found  to  contain  tin,  antimony, 
copper  and  arsenic.* 

THE    WORK    OF     UNDERGROUND    WATERS    IN 
PRECIPITATING   MINERALS. 

Are  minerals  precipitated  from  solution  in  concentrated  form? 
That  materials  are  deposited  from  underground  waters 


*  F.  Posepny,    'Genesis  of  Ore- Deposits,'   p.  43. 


CHEMICAL   GEOLOGY.  243 

we  know  from  actual  experience.  These  materials  include 
not  only  the  commoner  ones,  such  as  calcium,  sodium, 
silica,  etc.,  which  are  deposited  as  tufa  around  the  outlets 
of  springs;  but  also  the  less  common  elements.  The 
waters  that  bear  the  minerals  obtain  them  slowly,  picking 
them  up  here,  there,  and  everywhere,  but  it  is  not  every- 
where that  the  proper  conditions  occur  for  precipitation. 
The  result  is  that  where  the  favorable  conditions  do  occur, 
a  good  deal  of  the  materials  in  question  are  likely  to  be 
precipitated,  for  an  endless  supply  of  water  comes,  each 
bringing  and  contributing  its  mite.  In  this  way  concen- 
tration is  effected,  and  from  a  state  of  dissemination  in  the 
rock,  so  thinly  spread  that  the  most  expert  chemist  detects 
it  with  difficulty,  lead  may  be  concentrated  to  form  enor- 
mous bodies  of  galena,  or  gold  may  be  concentrated  to 
form  such  great  nuggets,  each  worth  a  fortune,  as  have 
been  found  in  Australia.  It  becomes  necessary  then  for 
the  investigator  to  gain,  so  far  as  he  may,  some  idea  of 
what  these  conditions  are. 

Why  are  ores  especially  apt  to  be  concentrated  along  water- 
courses? 

In  the  first  place,  it  has  been  pointed  out  that,  other 
things  being  equal,  ores  are  most  likely  to  occur  along 
water  channels,  such  as  a  fracture  or  set  of  fractures,  a 
porous  bed,  etc.  This  is  primarily  because  in  such  water- 
courses an  enormous  volume  of  water  continually  passes, 
and  to  a  less  degree  because  here  the  conditions  are  more 
favorable  to  deposition  than  elsewhere.  Into  unfractured 
or  otherwise  relatively  impervious  rock,  even  if  the  condi- 


244  GEOLOGY   APPLIED   TO   MINING. 

tions  for  deposition  are  of  the  best,  only  a  small  amount  of 
water  attains,  and  consequently  not  enough  metals  are 
ever  brought  to  form  an  ore-deposit.  But,  where  the 
conditions  favorable  for  deposition  occur  along  a  water- 
course, then  the  supplies  of  materials  are  so  great  that 
eventually  a  large  body  of  metallic  minerals  may  accu- 
mulate. 

MANNER  OF  DEPOSITION  IN  THE  DEEPER  UN- 
DERGROUND   REGIONS. 

In  what  forms  are  metals  usually  deposited  in  the  deeper 

underground  zone? 

Most  of  the  metals  are  deposited  in  the  deeper  region 
(and  to  a  great  extent  also  in  the  shallow  underground 
region)  as  sulphides.  Under  special  conditions,  other 
compounds  are  precipitated,  such  as  carbonates  and 
silicates.  The  deposition  of  zinc  silicates  at  Franklin 
(New  Jersey)  is  an  illustration  of  the  latter.  Metals  may 
also  be  precipitated  in  the  native  form,  as  is  probably  often 
the  case  with  gold,  platinum,  copper,  etc.;  they  may  form 
rarer  combinations  such  as  arsenides,  tellurides,  etc.,  or 
they  may  be  deposited  as  oxides. 

Under  what  conditions  are  r,netals  precipitated  as  oxides  in 

the  deeper  underground  regions? 

We  generally  think  of  the  formation  of  oxides  as  charac' 
teristic  of  the  surface,  and  of  the  sulphides  as  the  natural 
products  of  the  deeper  regions.  This  is  true,  as  a  rule,  for 
sulphides  commonly  change  into  oxides  during  the  pro- 


CHEMICAL   GEOLOGY.  245 

cess  of  weathering.  But  the  opposite  extreme  of  con- 
ditions from  those  prevalent  near  the  surface,  the  most 
intense  heat  and  pressure,  and  the  presence  of  strong 
solutions  and  vapors  may,  also,  produce  oxides.  An  oxide 
of  iron,  hematite,  is  often  deposited  from  gases  in  volcanoes. 
Magnetite  and  hematite  are  also  found  in  the  contact- 
metamorphic  deposits,  formed  by  the  highly  concentrated, 
heated,  and  compressed  solutions  and  vapors  given  off 
from  cooling  masses  of  molten  rock. 

Within  molten  but  slowly  solidifying  rocks,  oxides  of 
iron  (magnetite  and  titaniferous  magnetite),  and  probably 
oxide  of  iron  and  chromium  (chromite,  chrome  iron),  etc., 
are  accumulated  to  such  an  extent  as  to  form  ore-bodies. 
At  Franklin  Furnace,  New  Jersey,  oxide  of  zinc  (zincite) 
and  oxide  of  iron,  zinc  arid  manganese  (franklinite),  have 
been  formed  under  conditions  of  great  depth,  heat,  and 
pressure,  as  indicated  by  the  uncommon  minerals  with 
which  they  are  associated — minerals  usually  found  in 
contact-metamorphic  ore-deposits — and  by  the  highly 
metamorphosed  condition  of  the  beds  in  which  they  lie.* 

Oxides  formed  under  these  conditions  may  be  associated 
with  sulphides  formed  at  the  same  time,  as  is  the  case  in 
some  contact-metamorphic  ore-deposits. 

What  causes  the  precipitation  of  sulphides  from  solutions? 

One  of  the  most  important  causes  is  the  presence  of 
organic  matter,  in  shales,  sandstones  or  limestones. 


*  J.  F.  Kemp,  'Ore-Deposits  of  the  United  States,'  p.  257. 


246          GEOLOGY  APPLIED  TO  MINING. 

How  does  organic  matter  cause  ore-deposition? 

First  and,  probably,  most  important,  the  carbon  of  the 
organic  matter  may  unite  with  the  soluble  sulphates  of  the 
metals,  and,  by  abstracting  the  oxygen  from  these  salts, 
cause  the  formation  of  sulphides  which,  being  relatively 
insoluble,  are  precipitated.  The  carbon  and  the  oxygen 
unite  to  form  carbonic  acid,  which  is  carried  on  by  the 
water  in  solution,  and  immediately  assists  in  the  dissolving 
power  of  this  water.  Second,  much  organic  material 
contains  sulphur,  and  by  decomposition  this  sulphur  may 
become  sulphuretted  hydrogen.  A  rotten  egg  is  a  familiar 
example,  emitting  the  disagreeable  peculiar  odor  of  this 
gas.  When  this  sulphuretted  hydrogen  comes  into  con- 
tact with  soluble  salts  of  the  metals  it  combines  with 
them  and  precipitates  the  salts  as  sulphides.  For  ex- 
ample, a  solution  carrying  copper  and  iron  chlorides  would 
be  precipitated  by  sulphuretted  hydrogen  as  chalcopyrite, 
hydrochloric  acid  being  the  other  result.  This  acid  goes  off 
into  the  water  and  aids  further  solution  and  concentration. 
'  Most  beds  containing  organic  matter  give  off  a  certain 
amount  of  sulphuretted  hydrogen.  In  fact,  certain  shales 
and  limestones  have  this  peculiarity  so  marked  that  the 
former  are  called  by  the  Germans  stink-shales  (stink- 
schiefer)  and  the  latter  by  ourselves,  fetid  limestones. 

What  other  causes  of  ore-precipitation  are  there? 

Another  important  motive  for  deposition  is  operative 
when  metallic  salts  in  solution  come  in  contact  with  rock 
minerals  with  which  they  can  combine  to  form  a  mineral 


CHEMICAL    GEOLOGY.  247 

compound  more  stable  and  less  soluble  than  that  already 
existing.  This  is  in  accordance  with  a  fundamental  law  of 
chemistry,  and  the  result  is  the  very  important  process  of 
replacement. 

In  what  manner  do  replacement  deposits  form? 

The  replaced  rock  is  dissolved  by  the  water  and  for  each 
particle  taken  up  a  particle  of  vein  material  is  deposited  in 
its  place.  Microscopic  study  shows  how  insidiously  the 
solutions  can  work  their  way  through  the  apparently  solid 
rock.  They  circulate  along  even  the  tiniest  cracks  and 
between  the  crystals.  From  there  they  penetrate  into  the 
interior  of  the  individual  grains,  first  following  the  cleavage 
planes.  The  rock  undergoing  displacement  may  be 
sprinkled  with  disconnected  crystals  of  the  ore,  the  channels 
by  which  the  ore-bearing  solutions  have  come  being  invis- 
ible. Finally  most  or  all  of  the  rock  may  be  removed, 
leaving  a  solid  ore-mass. 

How  can  one  recognize  a  replacement  deposit? 

From  the  condition  of  the  formation  of  such  a  deposit  one 
usually  finds  all  stages  from  the  rock  sprinkled  with  ore- 
minerals  to  the  solid  ore.  There  is  a  usual  absence  of 
banding,  and  the  ore-bodies  are  apt  to  be  irregular,  with 
ill  defined  boundaries.  Yet,  since  they  usually  follow 
some  water  channel,  they  may  have  in  one  or  more 
directions  definite  extensions,  and  so  may  be  classified 
from  the  standpoint  of  form,  either  as  disseminations, 
irregular  masses,  shoots  (pipes  or  chimneys),  or  veins. 


248          GEOLOGY  APPLIED  TO  MINING. 

Sometimes  the  ore-mineral  occurs  as  a  pseudomorph,  after 
one  of  the  original  rock  minerals — that  is,  it  has  the  peculiar 
crystal  form  of  that  mineral — which  is  conclusive  proof  of 
replacement.  One  may  also  sometimes  find  fossils  com- 
pletely changed  to  an  ore-mineral  or  even  to  a  native 
metal. 

Example:  The  lead-silver  deposits  of  Aspen,  Colorado, 
have  formed  chiefly  from  replacement  from  limestone  and 
dolomite.*  Mineralization  began  along  fractures  (often 
microscopic)  resulting  from  some  rock  movement,  and 
often  attendant  upon  actual  faulting.  From  the  fractures 
the  metallic  minerals  penetrate  the  adjoining  rock.  Fossils 
have  been  found  which  are  imbedded  in  the  ore,  or  have 
been  so  changed  as  to  form  a  part  of  the  ore.  Fig.  63 
shows  a  mass  of  pure  native  silver  from  Aspen.  In  this, 
part  of  a  perfect  fossil  shell  is  firmly  fixed.  At  another 
place  a  fossil  was  found  completely  turned  to  zinc  and  lead 
sulphides  and  carbonates. 

Are  replacements  always  in  limestones  or  dolomites? 

Replacement  deposits  also  occur  in  difficultly  soluble 
rocks,  like  quartzites  and  granites.  Large  ore-deposits  in 
such  rocks  chiefly  due  to  this  process  are  abundant;  and, 
even  in  many  deposits  where  the  ore  has  formed  in  pre- 
existing cavities,  large  or  small,  replacement  will  be  found 
to  have  been  a  very  important  auxiliary  process. 

Examples:  1.  The  formation  of  auriferous  quartz  veins 
by  replacement  of  schist  along  zones  of  crushing,  in  the 


*  Monograph  XXXI,  United  States  Geological  Survey,  pp.  200-236. 


CHEMICAL   GEOLOGY. 


249 

A. 


district  of  Otago,  New  Zealand,  is  described   by  T 
Rickard.* 

The  sketch  (Fig.  64)  covers  a  width  of  5  feet.  Instead 
of  forming  a  narrow  clean-cut  crack  or  fissure,  the  soft 
schist  is  traversed  by  a  crushed  zone  bounded  by  parallel 
fractures.  The  parts  between  the  two  lines  of  fracture 
form  the  beginning  of  the  "mullocky  reef"  of  the  Austra- 
lian miner, — that  is,  a  lode  carrying  a  large  proportion  of 
included  country  rock.  Percolating  waters  deposit  their 


Fig.  63.  Fossil  imbedded  in  native  sil-  Fig.  64.  Type  of  lode  in  Otago,  New 
ver,  Aspen,  Colorado.  From  J.  E.  Zealand.  The  crushed  zone  between 
Spurr.  aa  and  bb  may  be  entirely  replaced. 

After  T.  A.  Rickard. 

quartz  first  along  the  lines  of  parting,  and  we  get  a  twin 
system  of  veins;  and,'  if  the  action  be  continued  further, 
the  intervening  filling  is  also  silicified.  It  may  seem  a  long 
step  from  a  lode  where  the  larger  part  of  the  gold-bearing 
material  is  crushed  rock,  to  a  massive  vein  of  clear  aurif- 
erous quartz,  yet  the  difference  is  one  of  degrees  only,  being 
due  to  the  variable  extent  to  which  the  quartz  has  replaced 
the  rock. 


*  Transactions  American  Institute  Mining  Engineers,  Vol.  XXI,  p.  418. 


250          GEOLOGY  APPLIED  TO  MINING. 

2.  The  ore-deposits  of  Monte  Cristo,  in  the  Cascade  Range. 
Washington,  have  formed*  chiefly  by  replacement  of  the 
granitic  rocks  and  andesites  in  which  they  occur.  The 
mineralizing  waters  have  come  into  the  rocks  along  joint 
planes,  and  have  then  replaced  the  wall  rock,  frequently 
forming  bodies  of  solid  ore  (chiefly  sulphides,  such  as  iron 
pyrite,  arsenopyrite,  blende  galena,  etc.)  several  feet  in 
thickness.  From  the  joints  the  waters  make  their  way  into 
cracks  of  microscopic  dimensions,  and  so,  little  by  little, 
attack  every  part  of  the  rock.  Of  the  original  minerals  of 
the  granitic  rocks  (tonalite)  and  of  the  andesite  (these 
minerals  comprising  chiefly  quartz,  feldspar,  hornblende, 
mica,  magnetite  and  augite)  all  except  the  quartz  are 
decomposed,  and  new  quartz,  calcite,  pyrite  and  other 
sulphides  grow  gradually  in  their  place.  With  a  moderate 
proportion  of  sulphides  in  the  altered  rock,  it  becomes  an 
ore,  with  a  quartz  and  sometimes  a  calcite  gangue. 


Does  the  presence  of  metallic  minerals  bring  about  the  pre- 
cipitation of  others? 

Metallic  oxides  and  sulphides,  already  existing  in  a  rock 
or  vein,  may  act  as  precipitants  of  dissolved  metallic  salts. 
Sulphides  often  change  their  composition  in  doing  so,  and 
form  a  compound  richer  in  the  metallic  base  than  before. 
Thus  chalcopyrite,  a  mineral  containing  34.5  per  cent  of 
metallic  copper,  may  be  transformed  into  bornite,  con- 
taining 55.5  per  cent.  Iron  sulphide  (pyrite)  may  unite 
with  copper  solutions  to  produce  chalcopyrite,  etc.  In 
auriferous  quartz  veins  the  gold  is  very  commonly  con- 


*  J.  E.  Spurr,  22d  Annual  Report  United  States  Geological  Survey,  Part  II, 
pp.  831-33. 


CHEMICAL    GEOLOGY.  251 

tained  in  the  iron  pyrite,  which  in  many  cases  seems  tc 
have  acted  as  a  precipitant. 

Sulphides  may  also  induce  the  precipitation  of  other 
sulphides,  particularly  of  similar  sulphides,  without,  so  far 
as  we  know,  any  chemical  reaction. 

How  are  ores  precipitated  by  the  mingling  of  solutions? 

Since  different  water  currents  traverse  different  rocks, 
composed  of  various  elements,  and  are  subjected  to  different 
conditions  of  heat,  pressure,  etc.,  the  mineral  solutions 
contained  in  each  one  will  rarely  be  identical  with  those  in 
another.  Whenever  two  currents  meet,  therefore,  the 
mingling  of  the  solutions  must  bring  about  certain  chemical 
reactions  and  some  materials  are  likely  to  be  precipitated. 
Where  waters  containing  free  sulphuretted  hydrogen  meet 
others  containing  soluble  metallic  salts,  for  example, 
metallic  sulphides  will  be  precipitated.  There  are  a  host 
of  other  reactions  which  may  take  place.  This  is  the 
chemical  explanation  for  the  principle  of  intersections, 
denned  in  Chapter  IV. 

Example:  In  the  Newman  Hill  deposits,  at  Rico,  Colo- 
rado, as  described  by  J.  B.  Parish,  T.  A.  Rickard,  and  F.  L. 
Ransome,  the  ores  commonly  occur  chiefly  on  the  under 
side  of  "blankets/7  which  are  impervious  beds  interstrati- 
fied  in  the  shaly  sandstones  of  the  region.  These  imper- 
vious beds  have  originated  in  several  different  ways,  but  in 
every  case  consist  of  a  decomposed  mass  through  which  the 
ascending  mineralizing  solutions  were  unable  to  pass,  and 
so  spread  out  and  deposited  the  metals.  Yet  the  under 
side  of  these  blankets  is  not  uniformly  mineralized;  there 


252 


GEOLOGY  APPLIED  TO  MINING. 


are  certain  rich  shoots,  outside  of  which  the  ore  is  poorer 
or  is  wanting.  The  blankets  and  other  beds  are  cut  by 
lodes  or  fissure-veins.  These  veins  have  their  upward 
termination  at  the  blanket,  but  extend  indefinitely  down- 


Sandstone 
Sandy  shale 
Sandstone 

Black  shale 
Blanket 

Blanket  limestone 
Black  share 
Sandstone 
Sandy  shale 

Sandstone 

Sandy  shale 

Sandstone 
Sandy  shale 

Sandstone 
Sandy  shafe 

Sandstone 
Sandy  shale 

Fig.  65.  Diagrammatic  section  across  a  lode,  and  its  blanket  pay  shoot.    New- 
man Hill,  Rico,  Colorado.    Scale,  1  in. =13  ft.    After  F.  L.  Ransome.* 

ward.  The  upper  end  of  the  veins  is  capped  by  a  rich 
deposit  of  ore,  lying  in  the  blanket  (Fig.  65).  These  pay 
shoots  seem  to  constitute  striking  examples  of  ore-bodies 
at  the  intersection  of  circulation  channels  now  occupied  by 


*  22d  Annual  Report  United  States  Geological  Survey,  Part  II,  p.  291. 


CHEMICAL   GEOLOGY.  253 

the  vertical  lode  and  that  afforded  by  a  porous  zone  under- 
neath the  blanket.  Solutions  ascending  in  the  fissures 
not  only  found  their  upward  progress  barred  by  the  imper- 
vious shales,  but  entered  a  porous  zone  traversed  by  later- 
ally moving  solutions,  which  effected  the  precipitation  of 
the  ores. 

Are  ores  precipitated  by  decrease  of  temperature  and  pressure 
in  ascending  waters? 

In  proportion  as  underground  waters  become  hotter  and 
under  greater  pressure,  their  power  of  solution  increases; 
therefore  when  they  become  cooler  and  under  less  pressure, 
on  nearing  the  surface,  their  power  of  solution  decreases, 
and  they  are  obliged  to  precipitate  some  of  their  burden. 
This  statement  demands  certain  qualifications.  For 
example,  the  laws  of  solution  are  different  for  different 
materials;  and  a  temperature  which  is  most  favorable  for 
the  solution  of  a  certain  salt  may  not  be  so  favorable  as  a 
lower  temperature  for  the  solution  of  a  certain  other.  But 
the  general  principle  above  stated  holds  good,  and  is 
influential  in  determining  the  deposition  of  ores.  For- 
merly, indeed,  it  was  held  to  be  more  important  than  it  is  at 
present,  and  was  called  upon  to  explain  most  ore-deposition 
from  ascending  waters.  But  now  that  we  recognize  the 
great  importance  of  deposition  by  mingling  of  solutions, 
by  contact  with  organic  matter,  and  by  replacement  of 
rocks,  or  by  contact  with  already  precipitated  metallic 
minerals,  we  see  that  it  must  be  rather  rare  that  an  ore- 
body  is  formed  by  release  of  temperature  and  pressure  alone. 
Yet  in  many  cases  where  we  have  evidence  that  some  other 


254  GEOLOGY  APPLIED  TO  MINING. 

cause  has  been  active  in  ore-deposition,  it  is  probable  that 
this  also  has  been  an  important  factor,  without  which  the 
precipitation  might  not  have  taken  place  at  this  point. 

For  example,  a  rising  column  of  heated  and  compressed 
water  may  meet  with  solutions  of  a  different  nature  at  the 
intersection  of  another  circulation  channel;  but  no  precipi- 
tation takes  place.  Further  up,  where  the  waters  are 
cooler  and  under  less  pressure,  they  may  again  meet  solution 
similar  to  those  met  below,  from  another  channel,  and  here 
abundant  precipitation,  resulting  in  the  formation  of  an 
ore-body,  may  occur. 

Example:  The  most  familiar  example  of  deposition  by 
this  cause  is  in  the  masses  of  sinter  which  are  thrown  down 
at  the  mouths  of  hot  springs,  when  these  emerge  into  the 
cool  and  free  open.  These  deposits  consist  largely  of  silica 
and  carbonate  of  lime;  but  other  substances  occur  in  them 
in  greater  or  less  quantity.  Iron,  arsenic,  tin,  and  many 
other  metals  have  been  found  under  such  conditions. 

At  what  depth  below  the  surface  are  ore-deposits  formed 
mainly  by  ascending  waters? 

It  is  probable  that  some  ore-deposits  are  formed  by 
ascending  waters  veiy  near  or  practically  at  the  surface. 
Such  metallic  elements  as  the  waters  carry  in  solution,  and 
which  they  have  not  precipitated  lower  down,  can  hardly 
fail  to  be  precipitated  when  coming  into  surface  conditions. 
The  question  then  is,  how  deep  may  these  deposits  be 
formed?  On  that  point  we  have  evidence  in  different 
mining  districts  that  ore-bodies  may  be  formed  at  least 
two  or  three  miles  below  the  surface. 


CHEMICAL  GEOLOGY.  £55 

Example:  In  the  Tintic  mining  district,  Utah,  there  is  a 
series  of  older  sedimentary  rocks  which  in  Mezozoic  time 
were  lifted  above  the  sea  and  folded.  Soon  after  this  time 
the  ore-bodies  were  formed.  Measurement  of  the  thickness 
of  the  stratified  rocks  found  in  some  parts  of  the  region, 
but  worn  away  from  the  ore-bodies  now  being  worked, 
shows  that  these  ore-bodies  were  mainly  formed  at  least 
12,000  feet  below  the  surface.* 

SPECIAL  CHEMICAL  PROCESSES  OF  THE  SHALLOW 
UNDERGROUND  WATERS. 

Are  the  chemical  processes  of  the  shallow  underground  waters 
the  same  as  those  of  the  deep  underground  waters? 
The  chemical  processes  of  the  shallow  underground 
waters  (mostly  descending  from  the  force  of  gravity)  are 
usually  considerably  different  from  those  of  the  deep  under- 
ground waters  (mostly  rising  from  hydrostatic  pressure, 
heat,  contained  gas,  etc.);  but  this  is  not  always  true. 
From  the  mere  inspection  of  the  results  of  the  chemical 
action  of  a  given  current  of  water,  it  is  usually  impossible 
to  tell  the  source  or  the  direction  of  movement.  Sulphides, 
for  example,  are  deposited  equally  well  by  ascending,  by 
descending,  and  by  horizontally  moving  waters. 

What  are  the  peculiar  chemical  effects  of  shallow  waters? 

Waters  coming  from  the  surface  contain  certain  gases  in 
solution  derived  from  the  atmosphere  or  the  decay  of  vege- 
tation, which  gases  are  not  so  abundant  in  deeper  waters. 

*  Tower  &  Smith,  19th  Annual  Report  United  States  Geological  Survey, 
Part  III,  p.  715. 


256          GEOLOGY  APPLIED  TO  MINING. 

Therefore,  the  chemical  effects  may  also  be  different. 
Water  coming  from  the  surface  contains  more  oxygen  and, 
as  a  rule,  less  sulphuretted  hydrogen  than  do  the  deeper 
waters.  Surface  waters  attack  the  rocks  through  which 
they  pass,  and  take  much  of  the  material  into  solution. 
Silicates,  sulphides,  etc.,  are  decomposed  and,  in  part, 
altered  to  oxides  and  carbonates.  Some  of  these  changes 
involve  an  access  in  bulk,  by  the  addition  of  the  oxygen 
and  carbonic  acid  from  the  waters.  In  some  cases  this 
increase  is  so  great  as  to  cause  minor  foldings  in  pliable 
banded  rocks,  and  brecciation  in  rigid  ones.  Other  chemi- 
cal changes  involve  a  shrinkage,  the  amount  of  material 
that  passes  into  solution  being  so  large  that  the  net  result 
is  to  cause  the  rocks  to  contract. 

ZONES  OF  WEATHERING  OR  OXIDATION. 

What  is  the  process  of  weathering? 

At  the  surface,  where  this  contraction  is  most  active,  it 
results  in  disintegration  and  crumbling  of  the  rock;  further 
down,  in  a  loss  of  strength  and  cohesion,  in  enlargement  of 
openings,  and  in  the  development  of  cracks  (often  not 
actually  opened,  but  only  potential),  into  open  fissures. 

This  general  process  is  called  weathering,  from  being  so 
conspicuous  at  the  surface,  where  the  rocks  are  exposed  to 
the -atmosphere,  or  weather;  and  the  zone  of  rocks  affected 
by  it  (a  zone  roughly  parallel  to  the  surface  configuration) 
is  called  the  weathering  zone. 

Example:  In  a  tropical  country,  like  Brazil,  the  surface 
weathering  of  rocks  is  vastly  more  active  than  in  cooler 


CHEMICAL   GEOLOGY.  257 

climates.  Decomposition  to  a  depth  of  100  feet  is  common, 
and  sometimes  extends  more  than  300  feet.  The  daily 
range  of  temperature  sometimes  amounts  to  more  than 
100°  Fahrenheit.  These  changes  cause  the-  rocks  to  crack 
and  to  admit  moisture  and  the  acids  which  bring  about 
rock  decay.  The  hot  season  is  the  rainy  season,  and 
waters  falling  upon  the  hot  rocks  have  their  temperature 
raised  to  about  140°,  which  makes  them  more  efficacious; 
and  the  rainfall  is  very  large.  The  chief  acids  which  aid 
in  the  decomposition  are  carbonic  acid,  nitric  acid,  and 
especially  the  organic  acids  derived  from  the  decay  of  the 
abundant  plant  and  animal  life.* 

7s  the  weathered  zone  the  same  as  the  zone  of  oxidation? 

This  practically  corresponds  with  what  is  called,  espe- 
cially in  consideration  of  ore-deposits,  the  zone  of  oxida- 
tion, being  that  belt  where  the  highly  oxygenated  surface 
waters  have  altered  the  sulphides  more  or  less  thoroughly 
to  oxides,  carbonates,  etc. 

Are  chemical  processes  active  in  the  rearrangement  of  ores  in 

the  zone  of  oxidation? 

They  produce  very  marked  results.  Rocks  near  the  sur- 
face are  physically  shattered;  hence,  waters  gain  access  to 
every  portion.  The  shrinkage  resulting  from  the  first 
chemical  reactions  of  these  waters  carries  on  the  work. 
The  rock  crumbles  (unless  it  is  immediately  swept  away  by 
the  streams)  into  a  kind  of  coarse  sand,  and  the  waters  have 
an  opportunity  to  search  thoroughly  every  part  of  it. 
Much  of  its  substance  is  dissolved  and  re-precipitated  or 

*  J.  C.  Branner,  Bulletin  Geological  Society  America,  Vol.  VII,  pp.  255-314. 


258  GEOLOGY  APPLIED  TO  MINING. 

carried  away.  When  re-precipitated  it  may  still  be  near  the 
surface,  or  may  be  far  below.  In  any  case  the  groupings 
change.  Scattered  amounts  of  the  metals  may  be  con- 
centrated by  this  process. 

What  is  the  iron  hat  (iron  cap)  or  gossan  of  an  ore-body,  and 
how  is  it  formed? 

In  most  ore-bodies  iron  forms  an  important  part,  even 
when  the  chief  values  are  in  some  other  metal.  Many 
copper  ores,  for  example,  consist  wholly  or  in  part  of  chal- 
copyrite,  the  sulphide  of  iron  and  copper.  During  and 
after  the  weathering  and  oxidation  of  the  surface  parts  of 
these  deposits,  the  metals  other  than  iron  may  be  leached 
out,  carried  down  and  re-precipitated,  leaving  the  iron 
(with  quartz  and  other  gangue  minerals,  if  they  were  pres- 
ent in  the  original  deposit),  in  the  form  of  a  soft  yellow 
limonite,  or  sometimes  as  hard  brown  limonite,or  hematite 
(oxides  of  iron).  This  iron  covers  the  valuable  ore,  which 
is  found  by  sinking  through  it.  In  Germany  it  is  called 
the  iron  hat,  in  Cornwall  gossan,  and  in  America  generally 
iron  cap  (or  capping).  A  strong  iron  cap  is  a  favorable 
sign  of  a  large  ore-body  beneath. 

Example:  *At  the  Cobre  copper  mines,  in  the  province 
of  Santiago,  Cuba,  the  principal  lode  and  the  chief  courses 
of  the  ore  are  indicated  at  the  surface  by  spongy  quartz  and 
iron  oxide,  with  highly  colored  clays.  Immediately 
beneath,  or  in  the  clays,  were  found  oxides,  sulphides  and 

*  Professor  Ansted.       Quoted  by  Hayes,  Vaughan,  and  Spencer,  'Geolog- 
ical Reconnaissance  of  Cuba,'    1901. 


CHEMICAL   GEOLOGY.  259 

carbonates  of  copper.  Further  down,  these  all  change  to 
copper  pyrite  (sulphide  of  copper  and  iron),  which  maybe 
regarded  as  the  original  ore,  the  carbonates  and  oxides  of 
copper  having  been  derived  from  it  by  oxidation,  and  the 
iron  cap  formed  by  the  leaching  of  the  copper  out  of  the 
iron  and  quartz.  The  oxidized  zone  extends  down  nearly  a 
hundred  feet. 

PRECIPITATION  OF  ORES  AT  THE  SURFACE. 
How  are  ores  precipitated,  from  solution  in  surface  waters,  in 


Water  containing  iron  finds  its  way  into  swamps,  and  on 
standing  there  for  a  while  the  oxygen  of  the  air  combines 
with  the  iron  carbonate  in  solution.  Carbonic  acid  is 
evolved,  and  hydrated  iron  oxide;  the  latter  forms  a  scum 
on  the  surface  or  sinks  to  the  bottom.  Successive  pre- 
cipitates may  accumulate  till  a  considerable  deposit  is 
formed.  These  "bog-ores,"  as  they  are  called,  have  been, 
and  are  still,  much  exploited  for  commercial  purposes. 
They  include  not  only  the  ores  formed  at  the  present  day, 
but  those  of  past  ages.  The  latter,  like  coal  beds,  have 
been  covered  by  later  sediments  and  so  preserved  till  now, 
when  they  are  often  exposed  by  erosion. 

Example:  In  the  Three  Rivers  district,  Quebec,  Canada, 
extensive  deposits  of  bog  iron  ore  have  formed  and  are  now 
forming.  The  iron  contained  in  the  streams  is  deposited 
in  swamps,  streams,  and  lakes,  wherever  the  water  is  for  a 
time  stationary,  or  choked  with  vegetation.  Beginning  as 
a  light  film,  the  ore  gradually  accumulates  to  thick  crusts, 
and  in  course  of  time  a  very  considerable  amount  accumu- 


260  GEOLOGY  APPLIED  TO  MINING. 

lates,  so  that  in  places  it  is  dug  out  for  commercial  use. 
This  iron  has  been  used  since  1730.* 

Are  ores  precipitated  from  the  waters  of  lakes  and  oceans? 

Waters  entering  oceans,  lakes,  etc.,  carry  mineral  matter 
which  may  be  finally  precipitated  on  the  bottom.  In  con- 
fined lakes  or  inland  seas,  important  deposits  of  common 
salt,  gypsum,  magnesium  and  potassium  minerals,  etc.,  are 
formed.  In  the  sea,  manganese  solutions  are  precipitated 
on  the  bottom  in  concretionary  form,  as  has  been  proved 
by  dredging.  Much  of  the  commercial  manganese  has  this 
origin,  the  manganiferous  layers  of  old  sea  sediments  being 
attacked  by  land  waters,  subsequent  to  uplift  and  erosion, 
and  the  manganese  being  thereby  more  highly  concen- 
trated. Other  metals,  such  as  copper,  and  even  silver  and 
gold  (chemical  analysis  has  proven  the  presence  of  these 
and  many  more  in  common  sea  water)  are  probably  pre- 
cipitated in  small  quantities  in  the  sediments  accumulating 
in  the  sea  bottoms  and  along  the  shores;  and  these  slightly 
metalliferous  layers,  after  uplift,  may  yield  to  land  waters 
a  material  which,  after  further  concentration  by  them,  will 
form  workable  ore-bodies. 

Example:  In  the  Paleozoic  region  of  Georgia  occur  ores 
of  manganese,  which  are  found  in  connection  with  only 
three  formations — a  limestone,  a  dolomite,  and  a  quartzite. 
The  manganese  is  in  clays  residual  from  the  decay  of  the 
rocks,  and  has  evidently  been  concentrated  from  a  dis- 
seminated state  in  these  beds  or  overlying  strata  now 

*  R&mmd  by  J.  F.  Kemp,    'Ore  Deposits  of  the  United  States,'   p.  90. 


CHEMICAL   GEOLOGY.  261 

removed  by  erosion.  It  is  supposed  to  have  been  derived 
originally  from  silicates  in  crystalline  rocks,  from  which 
it  was  taken  by  streams  in  solution  to  the  sea,  where  the 
Paleozoic  strata  were  being  deposited,  and  was  precipitated 
in  them.  Long  afterwards,  when  these  beds  were  again 
part  of  the  continent,  surface  waters  dissolved  and  concen- 
trated the  manganese  so  as  to  form  ores.* 

Are  mineral  deposits  ever  formed  at  the  surface  by  the  evapora- 
tion of  underground  waters? 

A  special  and  peculiar  phase  of  the  circulation  of  water  in 
rocks,  and  especially  in  soils  close  to  the  surface,  is  depend- 
ent upon  the  surface  evaporation.  Where  evaporation  is 
strong,  the  surface  would  quickly  become  entirely  dry  were 
it  not  that  moisture  from  deeper  down  rises  to  take  the 
place  of  that  removed.  This  action  is  particularly  strong 
in  hot  and  arid  climates.  The  mineral  content  of  the 
evaporated  waters  is  left  on  the  surface,  forming  the  crust 
of  salt  or  "  alkali"  familiar  in  desert  regions.  In  natural 
hollows  or  basins  in  the  topography,  beneath  the  surface  of 
which  the  groundwater  accumulates  (even  though  it  is  not 
abundant  enough  to  stand  long  above  the  surface),  so  much 
material  is  brought  to  the  surface  that  the  incrustation  is 
often  of  economic  value.  Such  deposits  consist  chiefly  of 
salt,  borax  and  soda.  In  some  cases  it  is  possible  that 
useful  accumulations  of  certain  metals  may  be  formed  in 
this  way. 

Example:     In  western  Colorado,   deposits  of  uranium 

*T.  L.  Watson,  Transactions  American  Institute  Mining  Engineers,  Feb., 
1903. 


262          GEOLOGY  APPLIED  TO  MINING. 

and  vanadium  occur  in  Jurassic  strata.  The  chief  mineral 
is  a  vanadate  of  uranium  and  potassium,  called  carnotite. 
The  ore  occurs  disseminated  through  sandstone,  as  irregu- 
lar bunchy  pockets  in  this  rock,  or  along  the  contact  of 
sandstone  with  shale.  The  ore  bunches  have  the  appear- 
ance of  being  impregnation  deposits,  formed  by  solution 
along  planes  of  easy  circulation,  frequently  bedding  planes. 
The  most  interesting  fact  concerning  them  is  their  super- 
ficial character.  They  are  flat-lying  streaks  which  in  some 
cases  disappear  into  unmineralized  sandstone  when 
followed  only  a  few  feet  underground.  In  places  the  ore 
has  formed  along  crevices  plainly  due  to  recent  surface 
movement,  showing  it  to  be  not  only  superficial  but  very 
recent. 

It  is  supposed  that  the  ore  exists  in  very  small  amounts 
in  the  sandstone  and  that  the  surface  deposits  have  been 
concentrated  from  this  condition;  and  it  seems  extremely 
likely  that  this  concentration  has  been  effected  by  the 
strong  evaporation  of  a  semi-arid  climate,  continually 
removing  the  moisture  from  the  surface  and  leaving  the 
dissolved  contents  behind  in  the  rocks.* 

By  a  similar  process  of  evaporation  incrustations  may  be 
formed  in  caverns. 

Example:  Nitrates  are  frequently  found  in  cavern 
earths.  A  large  amount  of  saltpeter  (nitrate  of  potash) 
was  taken  from  the  Mammoth  Cave  in  Kentucky  during 
the  war  of  1812,  and  from  caverns  in  Alabama  and  Georgia 
during  the  Civil  War,  for  the  manufacture  of  gunpowder. 
Investigation  of  these  deposits  points  to  the  conclusion  that 
the  nitrates  were  brought  in  by  water  percolating  through 
the  soils  above  the  caves  and  were  deposited  on  the  floors. 

*  F  L.  Ransome,  American  Journal  Science,  Fourth  Series,  Vol.  X,   pp.  121- 
130. 


CHEMICAL   GEOLOGY.  263 

Currents  of  air,  passing  in  and  out  of  the  caverns,  removed 
the  water,  leaving  the  salts  in  the  cave  earth.  The  accu- 
mulation of  salts  occurs  only  in  caverns  where  the  inflow 
of  surface  water  does  not  exceed  in  amount  the  water 
removed  by  evaporation.  In  wet  caves  the  soluble  salts 
are  washed  onward  with  the  water  bearing  them,  and  so 
are  not  deposited.  Nitrates  deposited  under  overhanging 
cliffs  have  the  same  origin.  As  to  the  source  of  the  nitrates, 
vegetation  furnishes  continually,  during  its  decay,  a  small 
amount  of  nitric  acid.* 

Are  there  other  instances  of  the  precipitation  of  ores  from 
surface  waters? 

Waters  containing  phosphoric  acid,  derived,  for  example, 
from  the  dung  of  sea  fowls,  may  change  limestones  or  lime 
marls  lying  near  the  surface  from  lime  carbonate  into  lime 
phosphate,  of  great  value  as  a  fertilizer.  Numerous  other 
examples  might  be  cited.  In  gold  placers  there  is,  as  pre- 
viously noted,  certainly  some  solution  and  redeposition  of 
the  gold  by  the  surface  waters,  even  though  gold  is  com- 
paratively resistant  to  solution  in  general. 

Are  minerals  secreted  from  surface  waters  by  living  organisms? 

The  precipitation  of  lime  and  silica  from  solution  in 
sea  water  by  incorporation  into  the  shells  of  marine  animals 
is  of  vast  importance.  By  the  accumulation  of  such  shells 
on  the  sea  or  lake  bottoms  originate  the  majority  of  lime- 
stone deposits.  A  tiny  fresh  water  organism  that  makes 
its  shell  out  of  iron  has  been  discovered,  and  the  accumu- 

*  W.  H.  Hess,  Journal  of  Geology,  Vol.  VIII,  p.  129. 


264  GEOLOGY  APPLIED  TO  MINING. 

lation  of  these  has  been  held  to  be  important  in  forming 
some  iron-ore  deposits. 

Are  ores  precipitated  from  surface  waters  by  organic  matter? 

Precipitation  by  organic  matter  plays  an  important  part 
at  the  surface,  as  well  as  underground. 

In  the  sea,  in  a  certain  broad  zone  somewhat  remote  from 
shore  and  yet  not  in  the  greatest  depths,  the  precipitation 
of  silicate  of  iron  (glauconite),  is  accomplished  largely 
through  the  agency  of  organic  matter,  and  through  the 
accumulation  of  this  glauconite,  and  its  subsequent  alter- 
ation and  re-concentration,  iron  ore-deposits  have  been 
formed. 

Example:  In  eastern  Texas  are  found  beds  of  limonite 
(hydrous  oxide  of  iron)  associated  with  marine  glauconitic 
sands.  The  silicate  of  iron  was  formed  beneath  the  sea, 
probably  chiefly  through  the  agency  of  tiny  organisms, 
which  precipitated  the  iron  and  silica,  either  from  fine  mud 
washed  out  from  the  land,  or  from  the  solution  in  the  sea 
water,  or  both.  After  deposition  the  beds  were  lifted  up 
and  became  dry;  then  the  surface  waters  decomposed  the 
glauconite.  The  iron  was  changed  to  oxide,  and  on  further 
concentration  (by  the  surface  waters)  formed  limonite 
beds.* 

Metallic  gold,  in  placer  regions,  is  frequently  found  in 
grass  roots,  having  been  precipitated  there  by  the  reducing 
action  of  organic  matter.  Pieces  of  wood,  etc.,  in  placers, 
have  the  same  effect. 

*  R.  A,  F,  Penrose,  Jr..  First  Annual  Report  Texas  Geological  Survey. 


CHEMICAL   GEOLOGY.  265 

Example:  On  one  of  the  tributary  streams  of  the  Galliko 
river  in  Macedonia,  exceedingly  little  gold  can  be  got  from 
the  gravels,  and  what  is  obtained  is  very  fine,  but,  in  some 
localities,  if  the  grass  and  turf  over  which  the  water  occa- 
sionally flows  be  pounded  up  and  washed  in  a  gold-pan,  a 
much  larger  quantity  of  coarser  gold  is  obtained.* 

PRECIPITATION  OF  ORES  IN  THE  SHALLOW  UNDERGROUND 

ZONE. 

Turning  away  from  the  precipitation  of  ores  at  the  very 
surface,  let  us  look  at  the  facts  of  their  precipitation  in 
rocks  near  the  surface.  It  has  already  been  explained  that 
in  the  process  of  weathering  the  superficial  portions  of  the 
rocks  are  largely  taken  into  solution,  as  well  as  much  of 
the  rock  lying  within  a  moderate  distance  from  the  surface. 

CONCENTRATION  ACCORDING  TO  RELATIVE  SOLUBILITIES. 

How  do  the  different  solubilities  of  metallic  minerals  bring 

about  their  selective  concentration? 

Some  materials  are  more  soluble  than  others,  hence  some 
valuable  metals,  for  example,  are  taken  with  difficulty  into 
solut  on,  and,  when  in  solution,  are  not  carried  far  before 
being  precipitated.  Others  are  more  easily  soluble  and  are 
carried  further.  The  result  is  that  in  ore-deposits  which 
have  been  greatly  affected  by  weathering  and  the  accom- 
panying action  of  surface  waters  concentration  of  metals 
according  to  their  relative  solubilities  is  very  great. 

*  Observations  by  the  writer. 


266          GEOLOGY  APPLIED  TO  MINING. 

In  the  case  of  a  mineral  not  easily  soluble  how  may  concen- 
tration take  place? 

The  concentration  may  take  place  by  the  removal  in 
solution  of  the  more  easily  soluble  minerals  with  which  it 
was  originally  associated.  This  forms  residual  deposits, 
which  are  often  important.  For  example,  phosphatic  lime 
nodules  in  limestones  are  frequently  concentrated  at  or 
near  the  surface  by  the  removal  in  solution  of  the  more 
easily  soluble  carbonate  of  lime  in  which  they  were  em- 
bedded; and  often  only  this  surface  portion  can  be  worked, 
the  unaltered  portions  containing  too  small  an  amount  of 
phosphate.  Iron  carbonate  or  limonite  nodules  in  lime- 
stone are  concentrated  by  the  same  process  into  workable 
iron  ore  at  the  surface.  Outcrops  and  weathered  portions 
of  gold-bearing  veins  are  often  richer  than  the  unoxidized 
portions  below,  for  much  of  the  rock  has  been  removed  in 
solution,  while  the  gold  has  been  attacked  to  a  less  extent ; 
hence  the  percentage  of  gold  in  the  weathered  and  oxidized 
rock  is  greater  than  in  the  unaltered  portions. 

Example:  In  the  gold  belt  of  the  Blue  Mountains,  in 
eastern  Oregon,  the  gold-quartz  veins,  which  carry  free 
gold,  are  more  or  less  oxidized  to  a  depth  of  from  100  to  300 
feet,  and  this  zone  is  generally  richer  than  the  unaltered 
ore  below.  At  one  mine  (the  Sanger,  on  Eagle  creek,)  the 
uppermost  100  feet  showed  a  narrow  vein  yie'ding  $25  per 
ton,  while  below  the  vein  widened,  and  the  average  values 
were  reduced  to  $12  per  ton.* 


*  W.  Lindgren,  22d  Annual  Report  United  States  Geological  Survey,  Part 
II,  p.  611. 


CHEMICAL   GEOLOGY.  267 

Are  minerals  taken  into  solution  re-precipitated  in  concen- 
trated form? 

Concentration  by  solution  and  re-precipitation  is  a  com- 
mon process.  In  places  where  there  is  only  one  mineral 
of  importance  this  mineral  may  be  compactly  precipitated 
in  the  zone  of  surface  waters.  Great  iron  ore-deposits  have 
been  made  in  this  way;  and  concentrations  of  the  more 
valuable  metals  are  frequent. 

Example:  1.  In  the  Tintic  district,  Utah*  (described  by 
Tower  and  Smith),  there  is  a  good  example  of  the  enrich- 
ment of  ores  in  the  oxidized  zone.  The  ores  contained  in 
the  sedimentary  rocks  in  this  district  (mostly  limestones) 
are  completely  oxidized  to  a  depth  of  several  hundred  feet, 
and  partially  to  the  lowest  points  reached  in  the  mine 
workings.  Surface  waters  have  decomposed  the  original 
sulphides.  The  metals  thus  attacked  have  been  largely 
taken  into  solution  and  re-deposited  as  new  minerals.  The 
metallic  minerals  of  the  original  deposit  are  sulphides  and 
sulpharsenides,  principally  pyrite,  galena,  enargite  and 
silver  sulphide.  These  have  been  changed  to  oxide  of  iron, 
sulphate  and  carbonate  of  lead,  hydrous  arsenates,  and 
arsenites  of  copper,  oxides  of  copper,  native  copper,  chlo- 
ride of  silver  and  native  silver.  During  these  processes  the 
various  metals  have  largely  been  segregated,  and  form 
distinct  deposits,  so  that  there  are  great  bodies  of  ore  con- 
taining principally  lead,  or  copper,  or  silver. 

In  the  veins  in  the  igneous  rocks,  in  the  same  district, 
the  oxide  ores  carry  about  twice  as  much  silver  and  lead  as 
the  sulphide  ores,  there  being  nearly  a  corresponding 
decrease  in  iron  and  silica.  These  segregations  of  the 

*  19th  Annual  Report  United  States  Geological  Survey,  Part  III. 


268  GEOLOGY  APPLIED  TO  MINING. 

metals  result  from  differences  in  the  solubilities  and  stabili- 
ties of  the  various  minerals. 

2.  In  the  Red  Cliff  mining  district,  Colorado,  the  ores 
occur  at  two  distinct  horizons.  The  first  horizon  is  in 
Lower  Carboniferous  limestone,  beneath  an  intrusive  sheet 
of  rhyolitic  rock,  and  the  ores  are  replacements  of  the  lime- 
stone by  iron  pyrite  and  silver-bearing  galena  on  an  im- 
mense scale.  The  oxidation  of  these  sulphides  to  sulphates 
and  oxides  may  be  well  observed.  The  second  horizon  is 
from  200  to  300  feet  lower,  geologically,  on  the  top  of  a 
white  Cambrian  quartzite.  The  ores  are  smaller  in  volume 
and  more  irregular  in  distribution,  but  are  very  much 
richer.  They  are  fine  ochreous  material,  largely  basic  iron 
sulphate,  containing  silver  and  gold.  There  is  good 
ground  for  assuming  that  these  metals  have,  in  part  at 
least,  been  leached  from  the  ore-bodies  of  the  higher  horizon, 
by  solutions  of  iron  sulphate.* 

When  a  number  .of  different  metals  are  thus  worked  over  does  a 
definite  arrangement  result? 

Where  a  number  of  metals  are  attacked  by  surface 
waters,  the  result  of  their  differences  in  solubility  is  the 
formation  of  rough  mineral  belts.  These  follow  the  surface 
in  general,  and  each  is  characterized  by  a  preponderance  of 
certain  metals  or  minerals.  For  example,  in  deposits  con- 
taining lead,  zinc,  and  copper,  the  effect  of  descending 
waters  may  be  to  separate  the  metals  into  zones,  the  lead 
(galena)  being  above,  and  zinc  (blende)  below.  Fre- 
quently there  is  a  third  and  still  lower  one  characterized  by 


*  Franklin  Guiterman.  Proceedings  Colorado  Scientific  Society,  Vol.  Ill, 
1890.  Supplement;  S.  F.  Emmons,  'Geological  Excursion  to  the  Rocky 
Mountains,'  p.  417. 


CHEMICAL   GEOLOGY.  269 

copper;  and  a  fourth,  characterized  by  iron,  has  been 
observed.  There  are  irregularities  in  these  zones,  and  the 
different  minerals  commonly  occur  together,  even  in  the 
same  hand-specimen;  but  in  a  broad  way  they  are  often 
well  defined. 

SECONDARY  SULPHIDE  ENRICHMENT. 

What  is  the  meaning  of  the  term  secondary  sulphide  enrich- 
ment? 

The  working  over  and  re-concentration  of  an  earlier  ore- 
body  into  richer  sulphides  by  descending  waters  has  been 
called  secondary  sulphide  enrichment. 

What  is  the  secondary  sulphide  zone? 

In  many  regions  secondary  sulphides  occur  in  a  more  or 
less  definite  zone,  underlying  the  oxidized  zone  and  over- 
lying the  primary  ores  (generally  also  sulphides).  The 
metals  are  leached  out  of  the  oxidized  ores  by  descending 
waters  and  carried  downward  to  the  unoxidized  suphides, 
where  they  are  themselves  precipitated  as  sulphides,  often 
by  the  direct  influence  of  the  primary  ores.  Such  secon- 
dary sulphides  are  commonly  richer  than  the  primary  ones. 

In  the  cases  where  ores  are  precipitated  as  sulphides,  from 
descending  waters,  where  does  the  sulphur  come  from? 
The    decomposition    of    less    stable   sulphides    is   sup- 
posed to  furnish  the  necessary  sulphur.     Even  where  no 
large  body  of  older  sulphides  exists,  disseminated  sulphide 
of  iron  (pyrite)  may  occur  as  it  does  in  many  sedimentary 


270 


GEOLOGY  APPLIED  TO  MINING. 


and  most  igneous  rocks,  even  in  those  apparently  fresh. 
Other  possible  sources  of  sulphur  are  sulphur-bearing 
waters,  and  the  sulphur  frequently  present  in  sedimentary 
beds  containing  organic  matter.  Soluble  metallic  sulphates 
in  surface  waters  may  sometimes  be  reduced  to  sulphides. 

May  an  ore-body  be  formed  wholly  by  descending  wa'ers  where 

none  already  exists? 

In  the  case  where  no  older  deposit  exists,  ore  may  still 
be  .ormed  by  descending  surface  waters,  provided  that  the 
rock  through  which  the  waters  percolate  has  a  sufficient 
quantity  of  disseminated  minerals. 

Example:  The  formation  of  iron  ore-deposits  by  descend- 
ing waters  has  taken  place  in  certain  parts  of  central  and 


Fig.  66.  Deposition  of  iroii  ore  by  descending  waters,  in  the  bedding  and  joiut 
seams  of  limestone;  and  nodular  iron  ores  in  residual  clay  in  the  hollows 
of  the  limestone.    Section  at  the  Pennsylvania  Furnace  ore- 
bank,  Pennsylvania.    After  T.  C.  Hopkins. 

eastern  Pennsylvania.  The  ores  are  oxides,  largely  lirho- 
nite.  They  occur  as  rounded  or  elongated  fragments,  with 
residual  clay,  in  irregular  deposits  in  cavities  which  extend 
from  the  surface  down  into  beds  of  limestone,  clay  or  sand- 
stone (Fig.  66).  The  original  source  of  the  iron  is  in  Paleo- 


CHEMICAL   GEOLOGY.  271 

zoic  shales  and  limestones,  where  it  is  disseminated  in  the 
form  of  carbonate,  with  some  sulphide  and  silicate.  The 
segregation  of  the  diffused  iron  into  the  ore  lumps  is  brought 
about  by  descending  surface  waters.  The  metal  has  been 
dissolved  by  the  organic  and  carbonic  acids  of  these  waters, 
and  precipitated  in  concentrated  form,  in  its  present 
position,  in  part  as  oxide,  and  in  part  as  carbonate,  which 
has  subsequently  been  oxidized.  Weathering  of  the  rock 
leaves  the  ores  embedded  in  residual  clays.* 

Are  ore-bodies  formed  entirely  by  descending  waters  likely  to 
be  so  important  as  are  secondary  sulphide  enrichments? 
Except  in  the  case  of  iron  and  some  other  of  the  com- 
moner ores,  such  ore-masses  are  usually  smaller  and  leaner 
than  those  formed  by  secondary  enrichments.  Moreover, 
at  a  moderate  depth  below  the  surface  the  mineralization 
is  apt  to  fail  to  such  an  extent  as  to  make  the  deposit  un- 
workable. This  depth  varies  with  the  character  of  the 
rock.  Where  the  rock  openings  are  small  and  closely  set 
together,  the  belt  of  mineralization  will  follow  more  closely 
the  surface  and. will  often  extend  only  a  distance  of  several 
hundred  feet;  strong  fracture  zones  or  fissures,  however, 
may  carry  the  surface  waters  and  their  effects  locally  much 
deeper. 

FEATURES   OF  THE   PROCESS  OF  RE-CONCENTRATION   OF 
PRE-EXISTING  ORES  BY  SHALLOW  DESCENDING  WATERS. 

Are  the  ores  concentrated  by  shallow  descending  waters  all 
leached  from  the  present  oxidized  zone? 
Whether  or  not  the  concentration  by  descending  waters 

*T.  C  Hopkins,  Bulletin  Geological  Society  of  America,  Vol.  XI,  pp.  475-502. 


272          GEOLOGY  APPLIED  TO  MINING. 

acts  upon  the  previous  ore-body,  the  results  are  not  simply 
those  which  can  be  obtained  from  a  given  belt  of  oxidized 
rock  at  a  given  period.  In  most  places  thousands  of  feet 
of  rocks — often  several  miles — have  been  removed  by 
erosion  to  lay  bare  the  present  surface.  As  the  surface 
rocks  are  stripped  away,  they  may  contribute  a  part  of 
their  metallic  contents  to  the  rocks  below,  and  so  the  surf  ace 
zones  of  mineralization  continually  migrate  downward, 
keeping  pace  with  the  erosion.  The  result  is  that  we  may 
have,  below  the  oxidized  zone,  not  alone  the  concentrated 
metals  of  that  zone,  but  contributions  from  many  ancient 
oxidized  zones,  long  since  swept  away. 

Are  regions  with  the  deepest  zones  of  weathering  or  oxidation 
most  likely  to  be  attended  by  rich  concentrations  by  de- 
scending waters? 

This  consideration  leads  to  the  comparison  of  oxidized 
zones  and  zones  of  metal  concentration  by  surface  waters. 
Oxidation  is  a  slow  process.  Hence,  in  countries  of  little 
erosion  (which  are  necessarily  those  of  little  moisture) 
oxidation  has  plenty  of  time,  and  extends,  partially  at  least, 
to  great  depths,  in  spite  of  the  fact  that  atmospheric  waters 
are  the  chief  agents  of  oxidation.  In  countries  of  great 
rainfall  and  erosion,  the  zones  of  oxidation,  though  rapidly 
formed,  may  be  swept  away  as  rapidly,  so  that  it  hardly 
penetrates  below  the  surface.  Yet  in  the  latter  case  the 
zone  of  concentrated  ores  may  be  more  rich  than  where  the 
oxidized  zone  attains  unusual  development,  for  the  thick- 
ness of  rock  removed  in  a  given  time  by  erosion  is  many 


CHEMICAL   GEOLOGY.  273 

times  more  than  in  the  arid  region,  and  therefore  larger 
quantities  of  metals  are  brought  into  solution  and  made 
susceptible  of  re-precipitation  and  concentration. 

What  are  the  conditions  determining  the  concentration  of  ores 
by  descending  waters  in  the  surface  zones? 

In  different  cases  the  concentration  of  pre-existing  veins 
by  surface  waters  varies,  whether  the  newly-formed  min- 
erals take  the  form  of  oxides,  chlorides,  carbonates,  or 
sulphides. 

The  conditions  which  determine  this  variability  of  effect 
are  divisible  into  three  classes. 

1.  Relative  quantity   of  solutions   and   slope   of  land 
surface. 

2.  Chemical  and  physical  nature  of  ores. 

3.  Chemical  nature  of  solutions. 

How  does  the  relative  amount  of  rain-  and  snowfall,  and  the 
surface  slopes,  affect  this  process? 

The  factor  determining  the  relative  quantity  of  solutions 
is  climate.  On  this  depends  the  amount  of  moisture  pre- 
cipitated. Other  things  being  equal,  a  large  quantity  of 
water  will  do  more  work  in  dissolving  and  re-precipitating 
mineral  matter  than  a  small  quantity. 

Another  important  circumstance  is  the  relative  rapidity 
with  which  that  portion  of  the  waters  which  remains  on  the 
surface  wears  away  the  rocks.  This  condition  is  depend- 
ent, with  a  given  quantity  of  rainfall,  upon  the  surface 
slopes. 


274  GEOLOGY  APPLIED  TO  MINING. 

We  may  consider  the  chances  in  four  different  kinds  of 
country: 

Moderate  to  well-watered  country  with  steep  slopes. 
Moderate  to  well-watered  country  with  slight  slopes. 
Arid  country  with  steep  slopes. 
Arid  country  with  slight  slopes. 

What  are  the  chances  for  this  process  in  a  well-watered  country 

with  steep  slopes? 

In  a  well-watered  country,  the  level  of  ground  water  will 
be  high,  and,  as  this  corresponds  in  general  to  the  upper 
level  of  sulphides,  the  zone  of  oxidation  will  be  relatively 
shallow.  If  the  slopes  are  steep,  the  surface  will  wear 
rapidly,  so  that  the  oxidized  zone  may  even  be  removed  as 
fast  as  it  forms,  and  the  sulphide  zone  may  almost  or  quite 
come  to  the  surface. 

In  this  case  the  abundance  of  waters  will  tend  toward  a 
complete  rearrangement  of  metals  in  the  superficial  zone, 
but  the  rapid  wearing  away  is  apt  to  interfere  with  this 
process,  and  the  concentrated  metals  which  outcrop  at  the 
surface  are  largely  removed  and  lost. 

What  is  the  usual  effect  of  descending  surface  waters  in 

a  well-watered  country  with  slight  slopes? 

If  the  slopes  are  slight,  and  the  supply  of  water  abundant, 
the  oxidized  zone  will  be  well  marked  and  thoroughly 
altered,  though  relatively  shallow.  The  rearrangement  of 
metals  according  to  their  relative  solubilities  will  be  com- 
paratively complete.  Under  such  conditions,  gossan,  or- 
iron  capping,  is  common.  According  to  the  solubility  of  the 


CHEMICAL  GEOLOGY.  275 

minerals  and  metals  in  the  orginal  deposit,  the  gossan  may 
be  exceptionally  rich;  or  very  poor,  with  rich  ores  below, 
in  the  enriched  oxidized  ores  and  in  the  enriched  secondary 
sulphide  zone. 

What  are  the  characteristic  effects  of  descending  surface  waters 
in  an  arid  region  with  steep  slopes? 

In  an  arid  region,  where  the  supply  of  water  is  small, 
the  changes  wrought  are  characteristically  incomplete.  In 
the  zone  of  surface  alteration  oxidized  and  unoxidized  ores 
occur  side  by  side,  both  often  occurring  together  at  or 
near  the  outcrop.  Therefore  the  oxidized  zone,  and  the 
zone  of  secondary  sulphide  enrichment,  are  not  so  well 
defined  as  in  regions  of  heavier  rainfall. 

The  level  of  ground  water  being  low  or  wanting,  the  zone 
of  partial  oxidation  extends  far  down.  For  the  same 
reason,  the  secondary  sulphide  zone  is  apt  to  be  indistinct, 
and  sometimes,  perhaps,  not  separable  from  the  oxidized 
zone. 

In  sum,  the  degree  of  rearrangement  is  not  likely  to  be 
so  complete  as  in  a  well-watered  region,  but  the  enriched 
zone  is  likely  to  be  as  thick  or  thicker,  and  will  be  more  of 
the  oxidized  than  of  the  secondary  sulphide  nature. 

Where  the  slopes  are  steep,  the  occasional  rainfalls  or 
snow  meltings  have  great  power  to  strip  the  surface,  and 
carry  it  down  to  the  valleys;  and  if  this  stripping  is  not  so 
active  as  in  a  well-watered  country,  the  same  scarcity  of 
water  limits  the  rapidity  of  ore-concentration  in  the  surface 
zone.  Here,  then,  the  zone  of  surface  rearrangement  is  apt 


276  GEOLOGY  APPLIED  TO  MINING. 

to  be  relatively  not  so  thick,  and  at  the  same  time  is  incom- 
plete. 

In  an  arid  region  with  slight  slopes  what  are  the  effects  of 
descending  surface  waters? 

Where  the  slopes  are  slight,  in  an  arid  country,  the  wear- 
ing away  will  not  keep  pace  with  the  alteration  in  the  sur- 
face zone.  Hence,  in  the  course  of  time,  the  rearrangement 
of  minerals  will  extend  to  a  very  considerable  depth. 

What  are  the  most  favorable  conditions  for  oxide  or  sulphide 
concentration  near  the  surface? 

It  seems  that  great  precipitation  is  more  favorable  to 
this  result  than  aridity,  and  the  slight  slopes  to  steep  ones. 
Most  favorable  is  the  combination  of  slight  slopes  and 
abundant  precipitation;  the  combinations  of  abundant 
precipitation  and  steep  slopes,  and  of  slight  precipitation 
and  slight  slopes,  are  perhaps  equally  favorable  one  to 
another;  but  in  the  first  case  the  oxidized  zone  will  be 
slight,  and  the  secondary  sulphide  zone  important,  and  in 
the  second  the  reverse  will  be  true. 

How  does  the  chemical  and  physical  nature  of  veins  affect  their 
superficial  concentration  by  descending  surface  waters? 

The  rearrangement  depends  upon  the  facility  with  which 
the  ores  are  taken  into  solution  and  re-deposited,  and  so 
easily  soluble  ores  should  be  more  quickly  and  completely 
rearranged  than  minerals  which  are  difficultly  soluble. 


CHEMICAL   GEOLOGY.  277 

Quartz  veins,  for  example,  containing  free  gold,  (which 
is  relatively  difficultly  soluble),  should  not  be  expected  to 
show  so  much  rearrangement  as  copper  ores,  which  are 
relatively  easily  soluble. 

The  physical  conditions  of  the  veins  also  largely  govern 
this  action  in  the  surface  zones.  If  the  metals  are,  from 
the  nature  of  the  vein,  easily  accessible  to  surface  waters, 
the  result  will  be  more  complete  than  if  they  are  not  readily 
attacked.  This  may  depend  on  the  original  characters  of 
the  vein,  or  on  subsequent  conditions.  For  example,  veins 
consisting  largely  of  metallic  minerals  are  much  more 
quickly  attacked  than  those  where  the  metallic  minerals 
are  small  in  amount  and  locked  in  gangue,  such  as  quartz. 
Also  veins  that  have  been  shattered  or  fractured  since  their 
formation  are  easier  of  attack  than  those  which  are  un- 
broken. The  actual  outcrop  of  a  vein  is  almost  always 
shattered  by  changes  in  temperature,  so  that  this  is  a 
specially  favorable  field  for  alteration  by  surface  waters. 


How  does  the  character  of  the  solvents  contained  in  descending 
surface  waters  affect  the  nature  of  ore-concentration  by 
them,  and  what  determines  their  character? 

The  character  of  the  solvents  is  often  as  important  as  the 
relative  solubility  of  the  metallic  minerals.  In  one  region 
where  a  given  solvent  is  present,  a  given  metal,  easily 
attacked  by  it,  may  be  readily  dissolved  and  re-deposited; 
in  another  case  the  same  metal,  for  the  Jack  of  such  solvent, 
may  remain  comparatively  little  altered.  In  descending 


278  GEOLOGY   APPLIED   TO    MINING. 

surface  waters  the  character  of  the  contained  solvents  may 
depend  on  the  nature  of  the  ores  in  the  vein,  on  the  nature 
of  the  gangue,  of  the  wall-rock,  of  the  soil,  or  of  surface 
deposits  of  various  kinds.  Through  all  of  these  descending 
waters  must  pass. 


EXAMPLES    OF    SECONDARY    ALTERATION     BY     SURFACE 
WATERS. 

1.  Gold  quartz  veins  in  a  country  of  steep  slopes  and 
abundant  precipitation.  Gold  belt  of  the  Blue  Mountains 
of  eastern  Oregon.* 

Typical  gold-quartz  veins  (that  is,  quartz  veins  contain- 
ing gold,  which  is  generally  associated  with  iron  pyrite 
scattered  through,  and  embedded  in,  the  quartz)  are,  it 
seems,  not  always  easily  affected  by  surface  waters.  In 
the  first  place  the  great  mass  of  quartz  protects,  to  a  large 
extent,  the  relatively  small  quantity  of  pyrite  from  the 
air  and  surface  waters. 

In  a  moist  country,  where  oxygen  and  carbonic  acid  are 
the  chief  reagents  contained  in  the  waters  which  sink 
below  the  surface,  gold  is  little  affected.  The  waters 
oxidize  the  pyrite,  and  the  dissolved  iron  is  carried  off. 
Ferric  sulphate,  which  may  be  one  of  the  products  of  the 
oxidation  of  the  pyrite,  can  dissolve  gold;  but  either  this 
action  is  slight,  or  the  gold  is  almost  immediately  precipi- 
tated again,  for  experience  shows  that  the  bulk  of  the  gold 
stays  in  the  free  state  in  the  oxidized  outcrops. 

It  even  remains  there,  where  erosion  is  very  weak,  after 
the  outcrop  has  crumbled  to  soil,  and  forms  residual  placer 

*Waldemar    Lindgren,    22d    Annual    Report    United    States    Geological 
Survey,  Part  II,  p.  611,  etc. 


CHEMICAL   GEOLOGY.  279 

deposits  (rooted  deposits).  Frequently  such  deposits  are 
rich., 

Where  the  slopes  are  steep  and  the  rainfall  abundant, 
as  in  the  Blue  Mountains  of  Oregon,  the  surface  debris  is 
swept  away  too  soon  to  permit  any  accumulation  of  the 
kind  above  mentioned.  The  water  level  is  high  in  this 
region,  and  oxidation  extends  down  from  100  to  300  feet, 
and  is  then  only  partial.  The  partially  oxidized  surface 
zones  sometimes  show  twice  as  much  gold  per  ton  as  the 
unaltered  lower  portions;  in  other  cases  there  is  very  little 
difference.  An  increase  of  gold  and  a  decrease  of  silver  in 
the  oxidized  zone  was  noted  in  one  case.  This  is  to  be 
explained  by  the  leaching  out  of  a  portion  of  silver  by 
oxidizing  waters.  Under  ordinary  conditions  silver  is  more 
easily  soluble  than  gold. 

There  is  no  observable  zone  of  enriched  secondary  sul- 
phides in  this  instance. 

2.  Veins  carrying  lead,  zinc,  copper  and  iron,  with  gold 
and  silver,  with  a  relatively  small  amount  of  gangue,  in  a 
region  of  great  precipitation  and  very  steep  slopes.  Dis- 
trict of  Monte  Cristo,  Cascade  -Range,  Washington. 

This  district  has  been  described  by  the  writer;*  and  the 
suggestion  made  that  the  ore-deposits  as  a  whole  may  have 
been  formed  by  downward  moving  solutions.  But  as  the 
waters  would  have  had  the  same  effect  upon  a  body  of 
earlier  ore,  formed  in  some  other  way,  the  case  still  serves 
to  illustrate  the  action  of  descending  surface  waters  under 
these  conditions. 

The  climatic  and  surface  conditions  are  practically  the 
same  as  those  in  the  Blue  Mountains  of  Oregon,  alreacjv 
cited.  But  while,  in  the  Blue  Mountains,  concentration  m 
the  partially  oxidized  gold  quartz  vein  is  slight,  and  a  zone 

*  22d  Annual  Report  United  States  Geological  Survey,  Part  II. 


280          GEOLOGY  APPLIED  TO  MINING. 

of  sulphides  formed  by  descending  waters  is  not  recognized 
as  existing,  in  the  Monte  Cristo  ores  the  sulphides  deposited 
by  descending  waters  form  remarkably  strong  and  com- 
plete zones.  The  difference  is  plainly  due  to  the  different 
characters  of  the  minerals  involved. 

In  the  Monte  Cristo  district,  as  in  the  Blue  Mountains, 
there  is  no  zone  of  complete  oxidation.  Sulphides  outcrop 
at  the  surface.  The  zone  of  even  tolerably  complete  oxida- 
tion does  not  extend  more  than  a  depth  of  ten  feet  any- 
where, and  generally  is  lacking. 

1  Enormously  abundant  surface  waters,  keeping  the 
ground-water  level  close  to  the  surface  most  of  the  year, 
have  dissolved  the  lead,  zinc,  copper,  iron,  silver,  gold,  etc., 
from  their  original  positions,  carried  them  down,  and 
re-precipitated  them  as  sulphides.  The  minerals  are  pre- 
cipitated roughly  in  the. order  of  their  relative  solubility,  the 
least  soluble  being  carried  the  least  distance.  Thus  the 
upper  zone  is  characterized  by  lead  (galena),  gold  and 
silver,  and  the  lower  limit  of  galena  follows  the  contour  of 
the  surface,  some  100  to  150  feet  below  it  (Fig.  67).  Below 
this  there  are  some  less  regular,  but  still  definite,  zones 
characterized  successively  by  zinc  (blende),  copper  (chal- 
copyrite),  and  iron  and  arsenic  (arsenopyrite  and  pyrite). 
The  sulphides  near  the  surface  carry  an  average  of  0.95 
ounces  gold  and  12  ounces  silver  to  the  ton;  at  some  dis- 
tance (a  few  hundred  feet)  from  the  surface,  the  pyrite  and 
arsenopyrite  contain  an  average  of  0.6  ounces  gold  and  7 
ounces  silver. 

A  maximum  of  600  feet  is  assigned  for  the  vertical  dis- 
tance between  the  surface  and  the  bottom  of  the  copper 
zone. 

3.  Copper  pyrite  ores  (or  iron  pyrite  carrying  some 
copper)  in  a  well-watered  country  with  moderate  slopes. 
Ducktown,  Tennessee,  etc.* __ 

*  W.  H.  Weed,   Transactions  American  Institute  Mining  Engineers,  Vol. 
XXX,  p.  449;  J.  F.  Kemp,  id.   Vol.  XXXI,  p.  244. 


CHEMICAL   GEOLOGY. 


281 


282  GEOLOGY  APPLIED  TO  MINING. 

The  general  conditions  permit  of  a  thorough,  though  not 
especially  deep  zone  of  oxidation,  and  a  high  water  level, 
below  which  sulphides  exist. 

The  solvents  or  reagents  in  such  surface  waters  are  chiefly 
(besides  the  water  itself)  oxygen  and  carbonic  acid.  Iron 
sulphide,  whether  free  (in  the  form  of  pyrite,  marcasite,  or 
pyrrhotite),  or  in  combination  with  copper  sulphides  as 
copper  pyrite  (chalcopyrite)  is  easily  attacked  by  the  oxy- 
gen of  surface  waters,  forming  iron  oxide  (limonite),  and  the 
sulphur  becoming  sulphuric  acid.  Iron  sulphate  is  also 
formed.  The  copper  sulphide  may  be  transformed  into  cop- 
per oxide  in  the  same  way,  but  it  generally  goes  into  solu- 
tion, chiefly  as  copper  sulphate,  and  passes  downward.  On 
reaching  unaltered  sulphides,  the  soluble  copper  salt  is  pre- 
cipitated, forming  copper-bearing  sulphides,  which  grow 
progressively  richer  with  the  continuation  of  the  process, 
the  iron  being  taken  away  in  the  form  of  the  soluble  sul- 
phate. Pure  copper  sulphides,  like  chalcocite  (copper 
glance),  is  even  formed.  The  iron  released  by  precipitation 
of  the  copper  sulphide  in  part  finds  its  way  further  down  in 
the  earth  and  is  there  again  precipitated,  as  pyrite. 

When  this  process  has  gone  on  for  a  long  time,  as  it  has 
in  this  case,  where  the  slopes  are  gradual  and  the  wearing 
away  does  not  move  faster  than  the  progress  of  the  altera- 
tion, there  will  be  a  surface  zone  where  the  oxidized  iron  ore 
(limonite)  will  be  left  with  the  quartz  of  the  original  gangue 
(made  spongy  by  the  dissolving  out  of  the  or'ginal  sul- 
phides), with  only  small  amounts  of  copper  carbonates  or 
oxides.  This  is  the  characteristic  gossan  or  iron  hat. 
Beneath  this  will  come  the  secondary  zone  of  very  rich 
copper  sulphides,  chalcocite  and  bornite,  with  pure  chal- 
copyrite. The  zone  is  apt  to  have  considerable  extent. 
It  constitutes  the  principal  ore-bearing  horizon  of  many 
,  copper  mines  of  the  kind  described.  Beneath,  the  pure 


CHEMICAL   GEOLOGY.  283 

copper  sulphides  will  disappear,  and  the  proportion  of 
copper  in  the  pyrite  will  grow  less  and  less,  till  the  original 
pyrite  with  a  small  percentage  of  copper — the  unaltered 
ore — comes  in  permanently. 

4.  Ores  of  lead,  zinc  and  copper,  with  silver  and  gold, 
in  an  arid  region  with  slight  slopes.  Horn  Silver  mine, 
Utah.* 

This  is  practically  the  mineral  composition  of  the  Monte 
Cristo  deposits,  but  the  climate  and  the  surface  conditions 
are  directly  reversed.  On  theoretical  grounds  it  has 
already  been  pointed  out  that  a  well- watered  country  with 
steep  slopes  and  an  arid  country  with  slight  slopes  were 
about  equally  favorable  for  the  concentrating  action  of 
downward  tending  surface  waters;  and  the  evidence  here 
seems  confirmatory. 

The  mine  is  situated  at  the  foot  of  a  mountain,  on  the 
edge  of  a  desert  valley.  The  footwall  is  limestone,  the 
hanging  wall,  andesite;  the  gangue  is  quartz  and  barite. 
The  workings  are  down  1,200  feet.  Oxidation  is  only 
partial;  galena  outcrops  on  the  surface,  mixed  with  lead 
and  sulphate;  yet  this  zone  of  partial  oxidation  extends 
down  to  the  lowest  depths  reached. 

Going  down  on  the  ore-body,  changes  in  the  ore  occur. 
The  upper  ores  are  lead  ores,  mainly  lead  sulphate  (angle- 
site)  with  sulphide  (galena),  some  carbonates  and  oxides 
of  lead,  and  horn  and  ruby  silver  (silver  chloride  and 
sulphide  of  silver  and  antimony).  No  zinc  and  copper  are 
found.  This  class  of  ore  persists  down  to  400  feet;  at  400 
and  500  feet  more  arsenic  and  antimony  are  found,  and  a 
little  zinc.  Further  down,  zinc  increases,  until  at  700 
feet  there  is  an  enormous  amount,  generally  in  the  form 
of  carbonate  or  silicate;  a  little  lead  is  associated  with  it. 


*  S.  F.  Emmons,  Transactions  American  Institute  Mining  Engineers,  Feb., 
1901. 


284  GEOLOGY  APPLIED  TO  MINING. 

At  650  feet  copper  begins  to  come  in,  and  extends  down  to 
750,  but  not  to  800  feet.  The  ore  is  largely  chalcocite 
(copper  sulphide)  with  a  good  deal  of  galena.  The  lower 
levels  contain  no  copper,  zinc  or  lead. 

The  results  are,  then,  practically  the  same  in  these  semi- 
oxidized  ores  as  in  the  sulphide  ores  of  Monte  Cristo, 
though  the  mineral  zones  are  somewhat  broader.  In  the 
Horn  Silver  mine  oxidized  minerals  and  sulphides  have 
evidently  been  deposited  side  by  side,  the  small  amount  of 
water  at  any  given  time  permitting  this.  For  example, 
ruby  silver  is  generally  in  such  cases  a  secondary  mineral, 
and  would  normally  occupy  a  deeper  zone  than  the  oxidized 
ores,  in  districts  where  the  supply  of  water  was  abundant 
and  there  was  a  definite  and  high  water  level.  In  the 
copper  belt  the  sulphide  chalcocite  is  probably  secondary, 
yet  it  occurs  with  -carbonates  and  silicates.  The  mineral 
zones,  therefore,  are  zones  of  partial  oxidation,  to  a  less 
degree  of  secondary  sulphide  deposition;  and  a  separate 
belt  of  secondary  sulphide  deposition  very  likely  does  not 
exist. 

The  chlorine  abundant  in  the  waters  of  dry  climates 
shows  its  effect  in  the  silver  chloride.  The  other  reagents 
were  probably  oxygen  from  the  air,  which  converted  the 
lead  sulphide  into  sulphate;  carbonic  acid,  very  likely 
derived  from  the  limestone  footwall,  which  produced  the 
lead  and  zinc  carbonates;  and  silica,  from  the  solution  of  the 
quartz  gangue,  which  produced  the  zinc  silicates. 

How  deep  does  the  zone  of  concentration  of  oxidized  or  sulphide 
ores  by  descending  surface  waters  generally  extend? 
In  the  Monte  Cristo  district,  Washington,  just  cited,  a 
maximum  of  600  feet  of  sulphide  enrichment  was  esti- 
mated, and  300  feet  is  probably  a  nearer  average.     In  the 
case  of  the  Horn  Silver  mine,  Utah,  the  concentrated  ores 


CHEMICAL   GEOLOGY.  285 

(mingled  oxidized  and  sulphide  ores)  extended  down  to 
750  feet.  At  the  De  Lamar  mine,  Nevada,  the  gangue  is 
quartz,  the  metallic  mineral  pyrite  and  perhaps  some  form 
of  telluride,  and  the  values  gold  and  silver.  The  enrich- 
ment extends  down  700  feet  or  somewhat  more.  The  ore 
here  is  all  oxidized.*  In  the  pyrite  deposits  of  southern 
Spain  and  Portugal  the  surface  zone  rich  in  copper  usually 
extends  some  300  feet  down,  below  which  the  pyrite  con- 
tains very  little  copper. f 

At  Ducktown,  Tennessee,  the  lower  limit  of  the  zone  of 
rich  copper  minerals,  formed  by  concentration  of  the 
original  lean  magnetic  pyrite  (pyrrhotite)  is  about  100  feet 
below  the  surface,  and  the  zone  is  thin,  the  iron  hat  or  gos- 
san occupying  the  greater  part  of  this  distance.  At  the 
Independence  mine,  Victor,  Cripple  Creek  district,  Colo- 
rado, the  zone  of  secondary  precipitation  and  enrichment 
of  ores  by  surface  waters  extends  in  general  some  400  or 
500  feet  below  the  surf  ace.  { 

MANNER  IN  WHICH  MINERALS  ARE  PRECIPITATED  BY 
DESCENDING  WATERS. 

In  what  forms  are  metals  usually  precipitated  by  shallow 

descending  waters? 

Deposits  by  descending  waters  may  be  oxides,  sulphates, 
carbonates,  chlorides,  sulphides,  etc.  Those  minerals 

*  S.  F.  Emmons,  Transactions  American  Institute  Mining  Engineers,  Feb., 
1901. 

t  Klockmann,  Zeitschrift  fur  praktische  Geologic,  1895. 

t  T.  A.  Rickard,  Engineering  and  Mining  Journal,  Vol.  LXXIV,  No.  26, 
p.  850. 


286  GEOLOGY  APPLIED  TO  MINING. 

which  have  been  affected  by  the  oxidizing  process  are 
converted  into  some  compounds  containing  oxygen,  whether 
it  be  sulphate,  carbonate,  or  oxide,  even  if  they  were 
originally  sulphides. 

Copper  sulphide  in  the  oxidized  zone  will  be  largely 
converted  into  copper  carbonate  (malachite  or  azurite)  or 
cuprite  and  tenorite  (red  and  black  oxides).  Copper 
sulphate  may  also  be  formed,  but  being  soluble  in  water 
will  not  as  a  rule  be  precipitated,  but  will  be  held  in  solution 
until  by  some  reaction  the  copper  is  precipitated  in  another 
form.  But  sulphate  of  lead  is  relatively  insoluble,  hence 
this  mineral  (anglesite)  is  frequent  in  the  oxidized  zone  of 
lead  ores,  as  well  as  the  carbonates  (cerussite)  and  the 
oxides  (minium,  litharge,  etc.).  Chlorides  are  formed  in 
the  oxidized  zone,  by  the  actions  of  waters  containing 
chlorine  or  alkaline  chlorides  in  solution,  and  the  metallic 
chlorides  that  are  relatively  insoluble  are  precipitated. 
Silver  chloride  or  horn  silver  (cerargyrite)  is  a  familiar 
case.  Easily  soluble  chlorides  such  as  those  of  iron 
and  gold  are  not  found  to  any  great  extent. 

Under  what  conditions  are  ores  most  likely  to  be  deposited  as 
chlorides,  in  the  oxidized  zone? 

In  arid  regions  there  is  little  or  no  free  drainage  to  the 
ocean,  and  the  surface  and  ground  waters  are  largely 
removed  by  evaporation,  leaving  their  solid  compounds 
behind.  Chlorides  of  sodium  (common  salt)  magnesium, 
etc.,  form  part  of  this  residue.  They  have  been  leached 
out  of  the  decomposing  rocks  in  small  quantities,  or 


CHEMICAL   GEOLOGY.  287 

supplied  by  hot  springs,  but  on  continued  accumulation 
and  -  concentration  by  evaporation  become  important. 
Such  is  the  origin  of  the  salt  pans  and  alkali  flats,  as  well 
as  the  saline  lakes  which  occupy  the  depressions  of  desert 
regions.  In  these  arid  tracts  alkaline  chlorides  are  abund- 
ant in  the  shallow  underground  waters  which  percolate 
through  the  ores,  and  the  results  are  likely  to  be  chlorina- 
tion  of  the  metals,  the  precipitation  in  the  weathered  zone 
of  the  insoluble  chlorides,  and  a  more  or  less  thorough  dis- 
solving out  of  the  soluble  ones. 

Example:  In  the  dry  tracts  of  Arizona,  New  Mexico  and 
Nevada,  where  salty  incrustations,  due  to  the  causes 
sketched  above,  are  found  in  nearly  every  valley  depres- 
sion, the  chloride  of  silver  is  noticeably  abundant  and 
frequent  in  the  weathered  zone  of  ore  deposits.* 

What  causes  the  deposition  of  sulphides  by  descending  waters? 
Deposition  of  sulphides  from  descending  waters  is  often 
brought  about  by  the  reduction  of  soluble  metallic  salts  by 
contact  with  already  existing  sulphides.  For  example,  a 
copper  solution  coming  in  contact  with  crystallized  pyrite 
(iron  sulphide)  may  be  reduced,  so  that  the  sulphide  of 
copper  and  iron  (chalcopyrite)  results.  Renewed  copper 
solutions  acting  upon  this  chalcopyrite  may  change  it  into  a 
sulphide  richer  in  copper,  such  as  bornite.  Still  renewed 
solutions  may  change  the  bornite  to  chalcocite.  Chalcopy- 
rite contains  34.5  per  cent,  copper,  bornite  55.5,  and  chal- 
cocite 77.8  per  cent,  so  that  the  quantity  of  copper  is 
greatly  increased. 

*  R.  A.  F.  Penrose,  Jr.,  Journal  of  Geology,  Vol.  II,  No.  3. 


288  GEOLOGY   APPLIED   TO    MINING. 

Example:  Chalcocite  is  the  principal  ore  in  the  great 
copper  district  of  Butte,  Montana,  though  bornite  and 
enargite  are  common.  The  chalcocite  forms  coatings  on 
the  other  metallic  minerals  in  such  a  way  as  to  show  that 
it  was  one  of  the  latest  minerals  to  crystallize.  As  depth 
is  gained  the  percentage  of  pyrite  and  enargite  increases  in 
comparison  with  that  of  chalcocite,  so  that  while  the  first 
thousand  feet  of  ores  averaged  8  or  10  per  cent  copper,  the 
second  thousand  averaged  about  6  per  cent.  The  chalco- 
cite has  been  formed  by  a  chemical  reaction  between  copper 
sulphate  in  solution  in  descending  waters  and  the  iron 
pyrites  and  other  primary  sulphides  lying  below.  By 
imitating  the  conditions  in  the  mines,  chalcocite  has  been 
produced  artificially.* 

What  is  the  action  of  organic  matter  in  the  shallow  under- 
ground zone,  as  regards  the  precipitation  of  sulphides? 
Precipitation  by  the  action  of  organic  matter  is  very 
important  in  the  shallow  water  zone,  both  near  the  surface 
and  at  considerable  depths,  and  takes  place  in  the  same 
way  that  has  been  described  for  the  deeper  underground 
regions.  Mine  timbers  (especially  those  in  old  mines)  may 
precipitate  metals  from  solution  in  mine  waters.  Dr. 
Raymond  has  reported  a  case  in  a  New  Mexican  mine, 
where  the  eye  of  an  old  pick  has  been  filled  with  galena 
(sulphide  of  lead)  by  the  reducing  action  of  the  wooden 
handle  which  once  occupied  this  position.  This  is  only  an 
example  of  what  must  often  occur  on  a  large  scale  when 
descending  solutions  come  in  contact  with  beds  of  shale  or 
other  sedimentaries  containing  organic  matter.  The 

*  H.  V.  Winchell,   Bulletin  Geological  Society  of  America,  Vol.  XIV,  pp. 
269-276. 


CHEMICAL   GEOLOGY.  289 

reactions  are  much  the  same  as  in  the  case  with  ascending 
solutions.     (See  p.  246.) 

Are  metallic  minerals  deposited  by  descending  waters  as 

replacements  or  as  cavity  fillings? 

In  respect  to  manner  of  deposition,  the  metals  borne  by 
the  shallow,  generally  descending,  waters,  may  be  precipi- 
tated in  the  same  way  as  those  in  the  deeper  waters.  Like 
them,  they  may  be,  and  perhaps  oftenest  are,  deposited  by 
replacement,  preferably  of  limestone,  frequently  of  some 
other  rock.  They  may  occupy  the  tiny  openings  of  a 
porous  rock,  or  cavities  formed  either  by  fracturing  or 
dissolution. 

CHARACTERISTICS  OF  ORE-DEPOSITS  FORMED  BY 
ASCENDING  AND  BY  DESCENDING  WATERS. 

7s  it  of  practical  value  to  know  whether  a  given  ore-deposit  was 
formed  by  ascending  or  descending  waters? 
This  knowledge  is  often  of  economic  importance. 

How  can  this  point  be  ascertained? 

Let  us  "take  a  case  where  metalliferous  solutions  are 
stopped  in  their  circulation  by  a  relatively  impervious 
stratum,  a  decomposed  dike,  or  other  rock  mass,  or  whatso- 
ever it  may  be,  and  where  as  a  consequence  of  the  spreading 
out  and  detention  of  the  solutions,  ore-deposition  takes 
place.  If  the  ore-deposits  are  conspicuously  placed  on  the 
under  contact  of  such  an  impervious  body,  it  is  a  fairly  safe 
index  of  ascending  currents;  if  on  the  upper  side,  of  descend- 


290          GEOLOGY  APPLIED  TO  MINING. 

ing.  The  case  is  emphasized  in  folded  strata,  for  the 
upward-tending  solutions  will  be  confined  chiefly  in  the 
tops  of  anticlines,  and  the  downward  moving  ones  in  the 
troughs  of  synclines,  and  here  the  ore-deposition  will  by 
preference  take  place. 

Example:  In  the  Bendigo  goldfield,  Australia,  described 
on  p.  165,  the  auriferous  quartz  veins  occur  by  preference  at 
the  apex  of  anticlinal  folds  in  the  stratified  beds  (saddle 
reefs)  (Fig.  32).  Deposits  in  the  synclinal  folds  (inverted 
saddles)  are  rare  and  unimportant. 

Where  ore-bodies  are  formed  at  an  intersection  of  circu- 
lation channels,  be  those  channels  joints,  faults,  porous 
beds,  or  any  combination  of  these,  they  will  often  be  found 
to  form  by  preference  either  on  the  upper  or  the  lower  side 
of  such  intersections.  They  will  be  either  in  the  troughs 
formed  by  the  two  channels  converging  downward  and 
meeting;  or  in  the  roof,  formed  by  the  channels  converging 
upward  and  meeting.  The  former  case  is  generally  an 
indication  of  descending  waters,  the  latter  of  rising  ones. 

Example:  The  typical  false  saddle,  auriferous  quartz 
veins  of  the  Bendigo  goldfields,  Australia,  drawn  by  T.  A. 
Rickard,  are,  as  shown  (Fig.  68),  formed  at  the  inter- 
section of  a  joint  a  a  with  a  bedding  plane.  The  fact  that 
the  ore-body  has  formed  in  the  roof  rather  than  in  the 
trough  of  the  intersection  may  be  regarded  as  indicating 
that  the  auriferous  quartz  was  deposited  by  ascending 
waters.* 


Transactions  American  Institute  Mining  Engineers,  Vol.  XX,  p.  469. 


CHEMICAL   GEOLOGY.  291 

When  ore-bodies  show  constant  and  evident  relation  to 
the  surface,  being  strong  on  the  outcrop,  but  shallow 
and  becoming  impoverished  with  depth,  it  is  an  evidence 
of  formation  by  descending  waters.  (Seep.  280 and  Fig.  67.) 

Example:  Many  iron  deposits  are  examples  of  this. 
The  Lake  Superior  iron  ore-bodies  are  due  to  descending 


Fig.  68.  Ore  formed  by  intersecting  fractures,    a  a  is  fracture  cutting  across 
stratification.     After  T.  A.  Rickard. 


waters,  as  is  shown  among  other  things  by  their  intimate 
connection  with  the  surface.  On  the  Mesabi  iron  range, 
near  the  north  shore  of  Lake  Superior,  in  Minnesota,  great 
masses  of  iron  ore  lie  at  the  surface,  under  glacial  drift 
(Fig.  69).  The  iron  was  originally  a  marine  precipitate, 
disseminated  through  a  sedimentary  rock,  from  which 
condition  it  has  been  concentrated  into  commercially 


292 


GEOLOGY    APPLIED    TO    MINING. 


valuable  ore-deposits,  in  favorable  places,  by  descending 
surface  waters. 

The  presence  of  cavities  crusted  with  stalactites  and 
stalagmites  of  ore  indicate  a  downward  movement  of  the 
waters  at  the  time  these  stalactites  and  stalagmites  were 
deposited,  and  very  likely  during  all  the  ore-deposition. 
Since,  however,  in  ore-deposits  formed  mainly  by  ascending 


Fig.  69.  Iron  ore-deposits  showing  constant  relation  to  surface  (formed  by  des- 
cending waters).    General  section  at  Biwabik,  Mesabi  iron  range. 
After  H.  V.  Winchell.* 


waters  there  are  apt  to  be  minor  stalactitic  growths, 
formed  by  downward  tending  surface  waters  of  a  later 
period,  short  duration,  and  relatively  slight  efficiency,  one 
should  guard  against  the  sweeping  application  of  this  test. 
Many  ore-deposits  are  formed  by  the  combined  effects  of 
ascending  and  descending  waters,  often  acting  at  different 

*  20th  Annual  Report  Minnesota  Geological  and  Natural  History  Survey. 


CHEMICAL   GEOLOGY.  293 

periods.  Ore-deposits  formed  by  ascending  waters  may, 
as  already  described,  be  worked  over  and  redeposited  by 
descending  waters.  Therefore,  one  must  beware  of 
applying  evidence  obtained  from  a  single  ore-body  to  all 
the  ores  of  a  district;  and  one  should  especially  avoid 
taking  the  evidence  of  the  shallow  ore-deposits  near  the 
surface,  where  there  is  frequently  strong  evidence  of  the 
work  of  descending  waters,  to  apply  necessarily  to  deeper 
ores. 

What  practical  deductions  follow  the  solution  of  the  question 
as  to  the  deposition  by  ascending  or  descending  waters? 

The  efficiency  of  downward  tending  waters  is  greater 
near  the  surface,  and  therefore  on  going  down  a  moderate 
distance  the  ore  deposited  from  such  waters  is  apt  to 
become  impoverished  and  fail  rapidly;  while  a  deposit  by 
ascending  waters  is  likely  to  be  much  more  deep-seated 
and  regular. 


CHANGES    IN   RICHNESS   IN    DEPTH. 

What  changes  do  ores,  deposited  by  descending  waters,  show 
in  depth? 

Ores  due  primarily  to  descending  waters  can  be  counted 
on  to  become  poorer  in  depth,  as  remarked  above.  Where 
an  earlier  ore-deposit  has  been  worked  over  and  concen- 
trated by  descending  surface  waters,  the  values  will  be 
often  less  below  the  enriched  shallow  zone. 


294  GEOLOGY    APPLIED    TO    MINING. 

What  general  changes  may  ores  deposited  by  ascending  waters 

show  in  depth? 

Concerning  the  great  class  of  ore-deposits  due  to  ascend- 
ing waters,  it  is  evident,  that,  being  limited  bodies, 
they  will  have  a  top  and  a  bottom  somewhere;  also 
that  at  some  point,  probably  intermediate  between  the 
top  and  the  bottom,  they  will  be  largest  and  probably 
richest.  These  ore-deposits  would  ordinarily  not  be 
revealed  to  the  eye  of  man  were  it  not  for  the  removal 
of  the  overlying  rocks.  Therefore,  the  wholly  fortuitous 
circumstance  of  the  level  of  the  plane  of  erosion  at  the  time 
of  the  discovery  of  the  ore-body  determines  whether  they 
will  become  stronger  or  weaker  in  depth.  Erosion  may 
reveal  only  the  top  of  a  deposit,  and  it  will  grow  richer 
below;  or  it  may  reveal  the  bottom,  and  it  will  grow  rapidly 
poorer;  or  it  may  cut  some  intermediate  level,  and  the  vein 
may  hold  its  own  for  a  long  distance  down,  with  about  the 
same  strength  and  richness.  These  conditions  are  apt  to 
be  fairly  uniform  over  a  considerable  district,  so  far  as  the 
ore-deposits  formed  at  a  single  period  are  concerned.  So 
in  one  district  the  ore  characteristically  grows  weaker  in 
depth,  in  another  stronger. 

What  changes  may  sedimentary  ores  show  in  depth? 

In  the  case  of  sedimentary  ore-bodies,  which  have  been 
folded  so  as  to  dip  at  a  high  angle,  it  is  plain  that  depth  can 
have  no  effect  in  determining  richness  or  poverty. 

How  deep  may  a  vein  extend? 
Under  favorable  circumstances,  where  there  is  a  strong 


CHEMICAL   GEOLOGY.  295 

water  channel,  and  constant  facilities  or  factors  favorable 
to  ore-deposition  all  along  it,  it  is  probable  that  the  resulting 
vein  may  extend  to  a  great  depth,  perhaps  in  extreme  cases, 
two  or  three  miles  below  the  original  surface.  Where  such 
a  vein  is  being  worked,  it  may  be  profitable  as  far  down  as 
the  present  methods  of  exploitation  can  be  pushed. 

Example:  The  gold-quartz  veins  of  Nevada  City  and 
Grass  Valley,*  California,  show,  in  general,  a  continuity 
down  to  the  greatest  depths  worked.  Many  of  the  smaller 
veins  diminish  and  disappear  in  depth;  but  the  larger  ones 
hold  their  own.  In  one  district  there  is  exposed,  by  mining 
and  irregularities  in  the  topography,  a  vertical  distance  of 
3,500  feet,  within  which  there  is  no  evidence  of  change  in 
the  character  or  quality  of  the  ore;  in  another  place  the 
same  truth  holds  for  a  vertical  distance  of  2,600  feet. 

These  veins  must  have  been  formed  at  a  depth  of  several 
thousand  feet  below  the  surface,  and  a  great  part  of  their 
original  extent  (the  upper  portion)  must  have  been  worn 
away  by  erosion. 

In  ore-deposits  due  to  ascending  waters,  may  the  character  of 
the  minerals  change  with  depth? 

There  may  be  a  change  in  the  character  of  mineral  in 
depth,  for  ores  may  be  deposited  according  to  their  relative 
solubility,  by  ascending  waters,  in  much  the  same  way 
(though  probably  not  so  regularly  and  definitely)  as  by 
descending  waters. 

*  W.  Lindgren,  17th  Annual  Report  United  States  Geological  Survey,  Part 
II,  p.  162. 


296  GEOLOGY  APPLIED  TO  MINING. 

Example:  In  the  case  of  the  Dolcoath  mine  in  Cornwall, 
which  has  probably  been  formed  by  ascending  waters, 
copper  is  relatively  more  abundant  in  the  upper  zone,  tin 
in  the  lower  one. 


May  ore-deposits  formed  by  ascending  waters  ever  extend 
great  distances  downward  without  change  in  the  character 
of  the  ore? 

In  some  cases  the  character  of  the  ore  may  remain  about 
the  same  through  a  considerable  vertical  range. 

Example:  The  silver-lead  deposits  in  Aspen,  Colorado, 
show  about  the  same  characteristics  through  a  known  verti- 
cal range  of  over  3,000  feet.  Here  the  ore-deposition  has 
been  largely  along  a  bedding  fault.  This  has  usually,  on 
one  side,  a  bed  of  shales,  which  have  probably  contributed 
toward  precipitating  the  ores  as  metallic  sulphides.  The 
beds  are  steeply  upturned,  so  that  for  considerable  depths 
uniform  conditions  for  deposition  have  obtained. 

Is  it  possible  that  any  veins  continue  downward  indefinitely? 

Owing  to  the  pressure  exerted  by  gravity,  it  is  doubtless 
more  difficult  for  a  fissure  to  stay  open  in  depth  than  near 
the  surface.  The  tendency  is  to  press  the  sides  together 
and  close  the  opening.  At  a  certain  depth,  it  is  probably 
the  case  that  the  pressure  and  the  plasticity  resulting  from 
this,  together  with  the  increase  of  heat,  makes  it  impossible 
for  fissures,  fractures,  or  other  openings  to  exist.  Such 
depth  has  been  variously  estimated  at  from  16,000  to 
33,000  feet 


CHEMICAL   GEOLOGY.  297 

Is  this  theoretical  downward  limit  of  any  practical  importance? 
This  limit  is  far  below  the  depth  that  can  be  attained  in 
mining.  But  some  veins  are  very  old,  and  even  the  com- 
paratively recent  ones  (such  as  the  Tertiary  veins)  are  old 
enough  to  have  had  in  many  cases  several  thousand  feet 
of  rocks,  which  overlay  them  at  the  time  of  their  formation, 
removed  by  erosion.  Hence,  it  is  very  possible  that  we 
may  find  some  veins  diminishing  and  even  disappearing  in 
depth. 

Why  is  it  that  veins  may  sometimes  diminish  in  size  and 
value  below  the  zone  of  oxidation? 

Near  the  surface,  as  already  described  (p.  182),  openings 
tend  to  become  large  and  numerous.  To  be  sure  of  the 
persistence  of  a  vein  which  has  depended  for  its  origin  upon 
a  fissure,  or  system  of  fractures,  one  must  first  explore  it 
down  below  the  zone  of  oxidation.  Many  a  vein,  large  and 
promising  near  the  outcrop,  will  be  found  to  dwindle  to 
quite  insignificant  proportions  before  arriving  at  this  point; 
but,  if  the  vein  is  still  strong  here,  there  is  good  reason  for 
expecting  that  it  may  continue  so  to  a  good  depth,  other 
conditions  being  favorable. 

Is  there  any  relation  between  the  horizontal  and  the  vertical 
extent  of  a  vein? 

There  are  all  kinds  of  fractures  and  fissures,  little  and  big, 
transitory  and  permanent.  It  is  a  saying  of  some  miners 
that  a  vein  will  go  down  as  far  in  depth  as  it  extends  hori- 
zontally on  the  surface  in  outcrop.  This  saying  seems  to 


298  GEOLOGY  APPLIED  TO  MINING. 

have  some  sound  principle  behind  it;  and  practically  the 
same  criterion  has  been  applied  by  W.  Lindgren  in  the  case 
of  the  gold  quartz  veins  of  Grass  Valley  and  Nevada  City, 
California.  Mr.  Lindgren  remarks:*  "In  considering  the 
probable  permanency  of  a  given  vein,  its  general  character 
must  be  taken  into  consideration.  Continuous  well- 
defined  outcrops  arid  wide  bodies  of  quartz  are  in  general 
good  indications  of  the  maintenance  in  depth,  as  is  also 
any  evidence  of  strong  faulting  and  movement.  .  .  . 
A  fissure  which  can  be  definitely  proved  to  extend  only  a 
short  distance  will  in  all  probability  be  found  to  be  corre- 
spondingly limited  in  depth." 

May  a  vein  change  in  value  in  depth,  on  passing  from  one 

rock  into  another? 

In  the  case  where  a  fissure  or  fracture  system,  along 
which  mineralization  has  taken  place,  passes  from  one  kind 
of  rock  into  another,  it  may  change  its  character  so  as  to 
influence  strongly  the  character  of  the  vein.  For  example, 
a  strong  fissure  in  limestone  may  die  out  entirely  on  meeting 
a  bed  of  shales.  This  is  an  influence  exerted  by  the 
mechanical  properties  of  different  rocks. 

Where  a  vein  or  other  ore-deposit  depends  largely  for 
its  existence  upon  the  chemical  properties  of  the  rocks 
through  which  it  passes,  the  same  changes  may  be  expected. 
For  example,  a  replacement  deposit  in  limestone  may  be 
expected  to  be  poor  or  to  stop  entirely  when  followed  down 
into  quartzite  or  granite,  although  the  physical  conditions 


*  17th  Annual  Report  United  States  Geological  Survey  Part  II,  p.  163. 


CHEMICAL   GEOLOGY.  299 

(the  fracture  zones  and  water  channels)  may  be  as  good 
in  the  latter  rocks  as  in  the  limestones. 

In  sum,  what  changes  may  be  expected  in  depth? 

There  is,  therefore,  no  general  rule  as  to  whether  veins 
and  other  ore-deposits  grow  richer  or  poorer  in  depth. 
Sometimes  they  may  grow  richer,  sometimes  poorer;  some- 
times they  may  grow  richer,  then  poorer,  then  richer  again: 
and  sometimes  the  distribution  of  values  may  be  fairly 
equable  to  a  considerable  depth.  Each  case  must  be  taken 
up  and  investigated  separately,  before  the  probabilities  can 
be  arrived  at. 

ASSOCIATIONS    OF   MINERALS. 

Can  the  presence  of  a  given  metal  in  an  ore-deposit   ever 

indicate  the  probable  presence  of  another? 

Owing  to  chemical  affinities  (the  fact  of  having  similar 
properties  of  solution  and  deposition,  etc.)  there  are 'asso- 
ciations more  or  less  marked  between  different  minerals  in 
ore-deposits;  and  this  association,  once  understood,  may  be 
of  practical  advantage  to  the  miner. 

What  are  some  of  these  associations? 

Lead  and  silver  are  closely  associated.  Most  silver  is 
obtained  from  argentiferous  galena,  and  most  galena  con- 
tains a  greater  or  smaller  amount  of  silver. 

Lead,  zinc  and  iron,  most  commonly  in  the  form  of 
galena,  blende  a'nd  pyrite,  but  also  in  other  forms,  are 
intimately  associated,  and  often  occur  together. 


300 


GEOLOGY  APPLIED  TO  MINING. 


Copper  and  iron,  whether  in  the  common  form  of  chalco- 
pyrite  (sulphide  of  copper  and  iron)  or  otherwise,  are 
closely  associated.  Pyrite  may  contain  a  variable  amount 
of  copper  sulphide,  increasing  up  to  the  pure  chalcopyrite 
(34.5  per  cent  copper). 

Lead  and  barium  (the  latter  in  the  form  of  barite,  or 
heavy  spar)  are  also  frequently  associated. 

Tin  and  a  number  of  rare  elements  are  usually  associated, 


Fig.  70.  Thin  sections  of  ore  from  Omaha  mine,  Grass  Valley,  California.     Mag- 
nified 17  diameters.    Light  areas,  quartz;  shaded  areas,  pyrite; 
black  areas,  gold.    After  W.  Lindgren.* 

though  in  any  given  case  (as  is,  indeed,  true  for  the  other 
associations  cited)  the  combinations  may  not  be  found. 
Among  the  metals  and  minerals  of  commercial  value  which 
often  occur  in  company  with  tin  are  tourmaline,  fluorite, 
topaz,  lithia  mica  and  wolfram. 

*  17th  Annual  Report  United  States  Geological  Survey,  Part  II,  PI.  V. 


CHEMICAL  GEOLOGY.  301 

Gold  is  frequently  found  in  veins  of  quartz ,  and  is  espe- 
cially associated  with  iron  pyrites  in  this  case  (Fig.  70). 

Do  non-metallic  minerals  form  associations  in  this  way? 

Another  class  of  minerals,  associated  because  they  are 
all  easily  precipitated  from  evaporating  waters,  are  salt, 
gypsum,  anhydrite,  and  frequently  borax,  etc.  These 
minerals  often  occur  also  in  red  rocks,  for  which  the  expla- 
nation is  probably  that  the  oxide  of  iron  which  colors  the 
rocks  was  also  a  chemical  precipitate  accompanying  evapo- 
ration. 

The  association  of  organic  minerals,  such  as  petroleum, 
mineral  asphalt  or  bitumen,  natural  gas  and  ozocerite 
(mineral  wax),  is  natural,  for  these  are  all  the  product  of 
the  various  distilling  and  other  processes  operating  upon 
sediments  rich  in  organic  mineral.  Wherever  one  of  these 
minerals  occurs,  others  of  the  same  group  should  be  sus- 
pected in  the  region. 

ROCK  ALTERATIONS  AS   GUIDE  TO  THE  PROS- 
PECTOR. 

What  guides  do  the  oxidation  processes  of  veins  afford  the 

prospector? 

The  chemical  peculiarities  of  ore  veins  and  associated 
rocks  may  often  furnish  the  prospector  a  guide  to  the 
presence  of  ore,  even  where  there  is  little  or  none  in  sight. 
Most  oxidized  outcrops  of  veins  are  marked  by  a  decom- 
posed mass,  colored  reddish  from  the  oxide  of  iron  which 


302  GEOLOGY  APPLIED  TO  MINING. 

has  been  formed  by  decomposition  from  the  iron  present 
in  the  unaltered  vein.  This  outcrop  may  look  like  any- 
thing but  a  metal  vein,  but  the  experienced  prospector 
recognizes  the  sign,  and  not  only  samples  and  assays  the 
decomposed,  often  clayey  material,  but  begins  making 
excavations  to  see  what  the  stuff  looks  like  a  little  further 
down.  Green  stains  on  such  an  outcrop  may  indicate 
copper,  nickel  or  chromium.  Bright  blue,  green,  and  red 
stains  together  usually  mean  copper.  Bright  red  stains 
may  come  from  either  lead  or  copper,  or  from  rarer  metals. 

Where  the  outcrop  of  a  quartz  vein  is  cavernous  or  cellu- 
/ar,  and  rusty  and  crumbling,  this  generally  points  to  the 
former  presence  of  iron  pyrite  and  other  sulphides,  which 
have  been  dissolved  out  by  the  weathering.  An  assay  is 
then  always  advisable,  for  in  the  dissolution  of  the  metallic 
sulphides  a  large  part  of  the  contained  gold  is  often  left 
behind,  so  that  this  sort  of  quartz  is  often  very  good  ore, 
and  it  is,  besides,  free  milling.  In  such  quartz  the  cubic 
cavities  are  often  exact  casts  of  the  dissolved  pyrite  crystals. 
In  depth  these  sulphides  are  reached,  and  the  ore  becomes 
more  refractory,  the  gold  which  nature  has  separated  in  the 
weathered  zone  being  difficult  of  separation  by  metallur- 
gical processes. 

What  indications  of  mineralization  are  afforded  by  certain 

rock  alterations? 

Less  well  known  to  the  prospectors  than  the  indications 
afforded  by  oxidation  are  those  given  by  the  alterations  in 
the  wall  rocks  which  often  accompany  ore-deposits.  In 


CHEMICAL   GEOLOGY.  303 

general,  if  a  rock  has  a  thoroughly  altered;  softened  or 
decomposed  appearance,  it  testifies  to  the  former  searching, 
dissolving  and  precipitating  effect  of  chemically  active 
solutions,  and  in  so  far  it  is  a  favorable  sign.  Rock  thus 
decomposed,  whether  it  was  originally  granite  or  quartzite, 
o-  anything  else,  is  often  called  by  the  general  and  incorrect 
term  porphyry,  especially  when  in  an  almost  clayey  state. 
An  abundance  of  iron  pyrite,  in  a  softened  igneous  rock, 
is  generally  a  good  sign. 

7s  this  rock  alteration  always  of  the  same  type? 

The  exact  nature  of  this  decomposition  and  alteration 
varies,  dependent  upon  the  character  of  the  rock  thus 
affected,  and  upon  the  chemical  nature  of  the  waters  which 
have  usually  done  the  work. 

Example:  The  ores  of  the  Silver  City  and  the  De  Lamar 
districts,  Idaho,  occur  as  quartz  veins,  carrying  frequently 
high  values  of  gold  and  silver.  The  wall  rocks  in  the  differ- 
ent mines  are  granite,  basalt  and  rhyolite.  There  is  good 
evidence  that  the  veins  were  deposited  by  ascending  hot 
waters.  The  granite  has  been  comparatively  little  altered 
by  these  waters,  the  basalt  more  so,  the  rhyolite  intensely. 
The  granite  is  often  nearly  fresh  close  up  to  the  vein,  though 
occasionally  it  is  softened  and  changed  by  the  alteration  of 
its  feldspar  to  pale  yellowish-green,  clayey  or  micaceous 
products  (sericite). 

The  basalt  is  altered  in  a  zone  along  the  vein,  which,  as 
a  rule,  is  not  wide,  but  of  very  irregular  extent.  In  the 
first  stages  of  alteration,  it  has  a  dull,  earthy,  dark-green 
color,  and  contains  small  cubes  of  pyrite.  When  further 


304          GEOLOGY  APPLIED  TO  MINING. 

altered,  the  rock  becomes  bluish-green  or  yellowish  (due 
to  the  development  of  the  mineral  epidote) . 

The  rhyolites  are  the  most  altered  of  all,  the  rocks  for 
many  hundreds  of  feet  from  the  veins  being  changed. 
The  chief  process  has  been  silicification,  so  that  the  rock 
often  keeps  its  hardness  and  has  a  deceptively  fresh  appear- 
ance. It  sometimes  contains  also  pyrite,  for  example,  at 
the  De  Lamar  mines,  where  this  mineral  is  very  abundant 
Sometimes  the  rock  has  been  softened,  and  near  the  veins 
some  streaks  have  been  entirely  converted  to  white  or 
yellowish  clay  (kaolin).* 

How  are  rocks  altered  near  tin  veins? 

In  the  neighborhood  of  tin  veins  in  granite  (the  usual 
occurrence)  the  rock  is  changed,  largely  by  the  removal  of 
its  feldspar,  and  the  deposition  of  some  new  mineral^ 
especially  white  mica  (muscovite),  in  its  place,  to  a  rock 
called  "greisen."  This  preserves  a  granitic  appearance, 
but  is  made  up  essentially  of  quartz  and  muscovite. 

What  does  the  silicification  of  rocks  signify? 

The  alteration  of  rocks  (preferably  limestones,  although 
to  a  less  extent  other  rocks)  to  silica  is  a  favorable  sign. 
The  resulting  rock  is  jasperoid.  Miners  call  it  quartz, 
quartz-rock,  flint,  chert,  hornstone,  jasper,  etc.  Ore- 
bearing  solutions  generally  contain  a  large  amount  of 
silica,  and  where  ores  are  deposited  (and  often  where  they 
are  not)  this  silica  replaces  the  country  rock  in  the  neigh- 
borhood of  water  channels.  Ore-deposits  are  not  in- 


*W.  Lindgren,  20th  Annual  Report  United  States  Geological  Survey,  Part 
III,  pp.  124,  177. 


CHEMICAL  GEOLOGY.  305 

frequently  surrounded  by  a  sort  of  rough  shell  of  such 
jasperoid. 

Example:  In  the  Tintic  silver-lead-copper  mining  dis- 
trict, Utah,  (described  by  Tower  and  Smith)*  the  most 
common  form  of  chemical  alteration  is  the  substitution  of 
silica  for  lime,  which  when  complete,  forms  jasperoid.  In 
this  change  the  structure,  texture,  and  color  of  the  original 
limestone  are  usually  retained.  The  jasperoid  is  not  only 
intimately  associated  with  the  ores,  but  forms  large  bodies 
which  have  little  or  no  metallic  content. 

*  18th  Annual  Report  United  States  Geological  Survey,  Part  III. 


CHAPTER  VL 
THE  RELATION  OF  PHYSIOGRAPHY  TO  MINING. 

What  is  physiography? 

Physiography  is  a  study  of  the  features  of  the  earth's 
surface  and  the  causes  by  which  they  were  made  to  assume 
their  present  shape.  By  this  study  we  arrive  at  the  com- 
prehension of  the  processes  by  which  mountains,  hills, 
valleys,  lake  basins,  etc.,  were  formed.  These  processes 
include  movements  of  the  earth's  crust,  both  faults  and 
folds,  and  the  erosion  of  rivers,  glaciers,  seas,  lakes,  etc. 
The  application  of  physiography  to  mining  is  perhaps  of 
limited  extent,  but  often  appears  unexpectedly. 

Which   are   more   favorable   to   ore-deposits,    mountains   or 

plains f 

In  general,  mountains  are  more  favorable  to  ore-deposits 
than  are  plains,  because  mountains  are  regions  of  disturb- 
ance. Here  the  rocks  are  usually  folded,  igneous  rocks  are 
likely  to  occur,  faults  and  fractures  are  developed  by  the 
movements,  and  all  conditions  necessary  to  ore-deposition 
may  be  present. 

In  plains,  on  the  other  hand,  the  rocks  are  more  likely 
to  be  undisturbed,  and  igneous  rocks  and  fracture  zones 
are  less  likely  to  occur. 


RELATION    OF    PHYSIOGRAPHY    TO    MINING.  307 

In  mountainous  regions,  also,  the  underground  water 
circulation  is  generally  more  vigorous,  on  account  of  the 
greater  differences  in  elevation.  Water  sinking  into  the 
rocks  at  the  top  of  a  steep  mountain,  for  example,  and 
emerging  at  the  foot,  will  circulate  vigorously  through  the 
intervening  space.  In  flat  countries  this  element  is  practi- 
cally absent. 

Are  ore-deposits  ever  found  in  plains? 

Some  plains  are  the  roots  of  old  mountains,  smoothed 
out  and  leveled  by  continuous  and  persistent  erosion;  and 
in  such  places  one  may  find  the  ore-deposits  that  were 
formed  in  the  old  mountains.  This  is  especially  likely  in 
the  case  of  such  veins  as  were  originally  deposited  at  a  con- 
siderable depth  below  the  surface,  such  as  gold  quartz  veins 
and  tin  veins. 

Moreover,  it  is  possible,  even  in  flat-lying  rocks,  that  the 
condition  of  circulation,  the  supplies  of  disseminated  metals, 
and  the  conditions  for  deposition,  may  be  so  favorable  as 
to  determine  the  formation  of  ore-bodies. 

What  is  a  fault  topography,  and  why  may  it  sometimes  serve  as 

a  guide  to  ore-deposits? 

There  is  in  each  district  a  distinct  type  of  topography, 
determined  by  the  natural  distribution,  relative  hardness, 
etc.,  of  the  stratified  and  igneous  formations  which  make  up 
the  rock  mass.  Faults  have  a  peculiar  effect,  in  breaking 
in  upon  this  type  of  topography  with  irregularities,  which 
are  partly  due  to  unequal  erosion  caused  by  lines  of  weak- 
ness developed  along  fault  zones,  and  partly  to  the  unequal 


308  GEOLOGY  APPLIED  TO  MINING. 

erosion  produced  by  the  checkering  of  rocks  of  different 
degrees  of  resistance,  by  the  faulting.  If  the  faulting  be 
complex  (especially  if  there  be  two  or  more  systems  of 
intersecting  faults),  it  may  produce  a  very  irregular  minor 
topography,  often  marked  by  low  rounded  hills,  and  very 
striking  to  the  eye  of  the  trained  geologist,  as  deviating 
from  the  conventional  type.  Such  districts,  especially  if 
they  are  in  a  region  where  igneous  rocks  occur,  are  fre- 
quently favorable  for  ore-deposits.  The  areas  of  complex 
faulting  may  often  be  regarded  as  the  outcrop  of  a  sort  of 
rock  column,  where  the  openings  are  so  large  as  to  induce  a 
concentration  of  the  water  circulation.  The  mining  camps 
of  Leadville  and  Aspen,  in  Colorado,  and  Pioche  and  Eureka, 
in  Nevada,  are  good  examples  of  this  kind. 

Single  faults  are  also  frequently  connected  with  ore- 
deposition,  and  may  be  traced  from  the  topography.  A 
cliff  or  scarp  may  result  from  the  direct  dislocation  of  the 
surface  by  a  fault,  or  from  the  unequal  erosion  of  rocks 
brought  together  by  it.  In  other  cases  the  fault  is  marked 
by  a  gully  or  valley. 

Do  veins  ever  produce  characteristic  forms  in  the  topography  f 

Sometimes  veins  are  harder  than  the  rocks  in  which  they 
occur,  and  in  this  case  erosion  leaves  them  as  ridges  of  an 
unmistakable  nature.  Such  is  often  the  case  with  quartz 
veins  and  other  hard  veins.  On  the  other  hand,  a  vein  may 
wear  away,  when  exposed  to  the  weather,  more  rapidly 
than  the  rock,  and  then  is  marked  by  a  straight  groove  in 
the  surface. 


RELATION   OF   PHYSIOGRAPHY   TO    MINING.  309 

Examples:  1.  In  the  Bendigo  gold  quartz  region, 
Australia,  the  quartz  veins  are  more  resistant  to  erosion 
than  the  enclosing  Silurian  slates  and  sandstones,  and 
hence,  are  left  at  the  surface  as  projecting  ridges  or  ledges.* 

2.  In  the  region  around  the  Copper  Queen  mine,  Arizona, 
a  relation  can  be  traced  between  the  surface  of  the  ground 
and  the  underlying  ore-bodies.  The  ore-bodies,  having  been 
eroded  faster  than  the  enclosed  rocks,  form  well-marked 
depressions.! 

When  may  a  knowledge  of  the  characteristic  erosion  forms  of 
different  kinds  of  rock  aid  in  locating  ore-deposits? 
Where  a  given  rock  in  a  district  is  known  to  be  preferably 
selected  for  ore-deposition  this  rock  may  often  be  recog- 
nized by  the  peculiar  topography  of  the  country.  For 
example,  a  limestone  weathers  quite  differently  from  a 
quartzite  and  produces  a  distinct  topography;  a  shale  or 
slate  is  again  different.  Eruptive  rocks  may  generally  be 
distinguished  from  sedimentary  by  the  difference  in  the 
topographic  forms  produced. 

How  are  deposits  of  soluble  minerals  sometimes  indicated  in 

the  topography? 

In  the  case  of  stratified  soluble  minerals,  especially  rock 
salt,  the  dissolving  out  of  portions  of  the  salt  bodies  under- 
ground by  circulating  waters  often  causes  a  series  of  de- 
pressions or  sinks  along  or  near  the  line  of  outcrop,  by 

*  T.  A.  Rickard,  Transactions  American  Institute  Mining  Engineers,  Vol. 
XX,  p.  318. 

t  James  Douglas,  Transactions  American  Institute  Mining  Engineers,  Vol. 
XXIX,  p.  537. 


310          GEOLOGY  APPLIED  TO  MINING. 

which  the  salt  zone  maybe  recognized.  Such  sinks  exist  In 
the  neighborhood  of  the  great  salt  deposits  of  Stassfurt, 
in  Germany,  and  the  settling  of  the  rock  due  to  the  under- 
ground caverns  has  often  involved  the  partial  ruin  of 
villages. 

Are  ore-deposits  more  likely  to  occur  at  the  tops  of  hills  or  at 

their  bases? 

Professor  Van  Hise*  has  explained  that  in  regions  where 
deposits  were  made  by  descending  waters  during  the  exist- 
ence of  the  present  topography  ore-deposition  is  likely  to 
be  greater  at  the  crests,  or  beneath  the  upper  slopes,  where 
the  quantity  of  descending  water  is  greatest.  Deposits  by 
ascending  waters,  he  explains  (always  restricting  this 
observation  to  such  ore-deposits  as  were  formed  during  the 
present  topography)  are  more  likely  to  occur  beneath  the 
valleys,  or  beneath  the  lower  slopes,  for  here  ascending 
springs  usually  emerge.  Third,  ore-deposits  which  have 
been  first  formed  by  ascending  waters  and  subsequently 
enriched  by  descending  waters  wftl  be  on  the  slopes,  prob- 
ably in  many  cases  nearer  the  valleys  than  the  crests. 

*  Transactions  American  Institute  Mining  Engineers,  Vol.  XXX,  p.  27  et.  seq. 


INDEX. 


INDEX. 


Page 

Acrogens,  defined 51 

Adirondack  Mountains,  magnet- 
ite      105 

ore-dikes 117 

Age  of  rocks  containing  ore-de- 
posits       65 

Age  of  rocks,  how  told.  .  .  42,  43,  56,  57 

Age  of  veins 63 

Alabama,  saltpeter 262 

Alaska,  coal 66 

glaciers 133 

Koyukuk  district 214 

Kuskokwim  River 122 

Nome 219,221 

placers 218 

Seward  Peninsula 210 

Yukon  River  .  .  122,  124,  129,  174 

Algae,  defined 51 

Alkali  flats,  origin 261 

Alterations   of   rocks,    guides   to 

prospecting 302 

Alum 109 

Aluminum,  ores I? 

oxide 106 

sulphate 110 

Alunile 109 

Ammonites,  defined 47 

Amphibians,  defined 48 

Amphibolite 11 

Amygdaloid 90 

Andesite,  alteration 91 

ores  deposited  from 63 

replacement  of 250 

Andesitic  rocks,  defined 87 

transitions 94 

Angiosperms,  defined 50 

Anglesite 286 

Anhydrite,  associated  minerals  . .   301 
Animal  kingdom,  division  of  ....      44 

Anogens,  defined 51 

Anorthosite,  defined 117 

Ansted,  Prof.,  cited 258 

Anticlinal  folds,  oil  and  gas  in  ...    166 

Anticlinal  mountains 127,  128 

Anticlines 121 

apex  of 165 

fracturing  in 167,  168 

ore-deposition  in.  164,165,167, 168 

Antimony  in  springs 242 

Italy 63 

Mansfeld,  Germany 70 

Antimony  sulphide,  deposited  by 

fumaroles 110 

Apatite  in  veins 109 

Apex  9f  anticlines 165 

Aqeo-igneous  fluidity 13 

Archaean  granites 95 

rocks,  characteristics  of  ....      37 
veins  in 63 


Archaean  period. 37 

Arches  in  strata' 166 

Argentine  Republic,  oil 69 

Arid  climates,  superficial  altera- 
tion of  ores 283 

Arid  region,  chlorides  in 286,  287 

Arizona,  Copper  Queen  mine  ....    309 

Tombstone 168 

Arkansas,  lead  and  zinc 194 

Arsenates,  derived  from  arsenides  267 

Arsenic  in  sinter 254 

in  springs 242 

with  copper 267 

Arsenic  sulphide 110 

deposited  from  fumaroles ...      17 

Italy 64 

Arsenides 244 

Arsenites,  derived  from  arsenides  267 

Arsenopyrite.  auriferous 206,  280 

Articulates,  defined 47 

Ascending  solutions,  ore-deposits 

by 19,  74,  165.  166,  168 

Ascending     water     ore-deposits, 

changes  in  depth 

.  .  .  289,  290,  291,  294,  295,  296 

criteria 289,290,291 

Aspen,  Colorado.  169,170,202,203,  248 

age  of  ores 63 

parting  quartzite 55 

Asphalt  mineral,  origin 301 

Association  of  minerals 299,  301 

Atmospheric  waters,  ore-deposi- 
tion by 189 

Augite  in  diabase 100 

in  rocks 7 

Auriferous  quartz-veins 77 

Australia,  Ballarat 77 

Bendigo 

164,  165,  167, 186,  188,  289,  309 

Omeo 13 

Tertiary  placers 65 

Australian  gold,  age  of  containing 

rocks 62 

Azurite 286 

B 

Baku,  oil-bearing  strata 69 

Ballarat,  Australia 77 

Banded  structure,  igneous  rocks  .  79 

metamorphic  rocks 4 

veins 190,  191 

Bars,  formation  of 217 

Bar  placers 216 

Barite 300 

as  gangue 283 

Basalt,  alteration 91 

altered  to  greenstones 10 

Columbia  River 98 

columnar  jointing 174 

gold  and  silver  in 101, 102 


314 


Basaltic  rocks,  defined 

transitions 94 

Basaltic  structure 174 

Basic  igneous  rocks,  gold  and  sil- 
ver in 103 

Basic  rocks,  connection  with  cer- 
tain metals 113,  114 

connection  with  ore-deposits.   112 

Basic,  term  defined 11,  104 

Basin,  structural 148 

Bassick  mine,  Colorado 110 

Bauxite 8 

Beach  placers 218,  219 

fossil 227 

reconcentrated 219,  220 

seaward  extent 220 

Becker,  G.  F.,  cited.  . 240 

Bed,  defined 27 

Beds  of  ore,  association  with  cer- 
tain rock-characters 60 

identification  by  fossils 68 

primary  and  secondary 59 

Bed  form,  minerals  occurring  in .  .      58 
Bedded  deposits,  subsequent .... 

71,73,74,75 

Bedded  ores,  precipitation 71 

Bedding,  explanation 27 

faults 158,159 

Bed-rock,  of  placers    223 

placer  gold  on 208,  209 

Bench  placers 221 

Ben  More,  metamorphism 3 

Birds,  defined. 48 

Bismuth,  sulphide 17 

telluride 17 

Bittner,  cited 129 

Bituminous  shale 69,  70 

Black  sand  in  gravels 210 

Blake,  W.  P.,  cited 70 

Blende  in  coal-shales '73 

deposited  in  mine  workings  .      64 

zone  near  surface 268,  280 

Bog  iron  .  . 241,  259 

Borax,  associated  minerals 301 

formed  by  evaporation 261 

Bornite 282 

formation 250,  287 

Boron,  minerals  containing 108 

Brachiopods,  defined 47 

Branner,  J.  C.,  quoted 257 

Brazil,  weathering 256,  257 

Breccia,  defined 92 

due  to  chemical  charges  ....    256 
British  Columbia,  platinum 114 

Tertiary  placers 65 

Brooks,  A.  H.,  cited .  210,  220,  221,  227 

Browne,  R.  E.,  cited 223 

Butte  ore-deposition 116 


Cadell,  H.  M.,  cited 3 

Calamites,  defined 51 

Calcite  in  contact  metamorphic 

deposits 108 

in  dolomite 30 

in  metamorphic  rocks 17 

California,  auriferous  gravels.  .  .  .  224 

gold  in  rocks 101 


INDEX. 


California,  Grass  Valley.  100,  124,  300 
Nevada  City  and  Grass  Val- 
ley   197, 295 

old  placers 223 

Tertiary  placers 65 

Cambrian 37 

fossils  of 38 

lack  of  coal  in 67 

origin  of  term 36 

placers 65 

rocks 43 

Cambrian  period 37 

Cambrian    sediments,    metamor- 
phism of 3 

Canada,  magnetite 105 

Three  Rivers  district 259 

Carbonate  of  iron  deposits,  origin.   266 
Carbonate  of  lime,  deposition  at 

surface  . 25 

Carbonates,  deep  formation 244 

formation  at  surface 256 

Carbonic  acid,  in  rock-weathering  257 

in  waters 240 

Carboniferous,  coal 66 

fossils  of 39 

gold  in .62 

origin  of  term 36 

rocks 43 

Carboniferous  period 37 

Carmichael,  Mr.,  cited 114 

Carnotite 262 

Carpathians,  oil 69 

Cassiterite 109,  113,  230 

Castro,  Fernandez  de,  cited 114 

Cavern  deposits 75,  262 

Cavities,  ore-deposition  in ".    190 

Cavity  filling 33 

Cephalopods,  defined 46 

Cenozoic  era 37 

Cerargyrite 286 

Cerussite 286 

Chalcocite 287 

formation 282,  2M8 

secondary 234 

in  Sierra  Oscura 72 

Chalcopyrite,  alteration  to  born- 


ite 


deposited  by  fumaroles. 
formation  .  . 


250 
110 
287 
288 
280 
12 
243 


formed  from  pyrite 

zone  near  surface 

Changes  in  depth,  ores 

Channels,  ore-deposition  along  .  . 
Chemical  action  of  surface  waters 

20,21 

Chemical  action  of  underground 

waters 22 

Chemical    agencies,     ore-concen- 
tration by 236,  237 

Chimneys  (ore) 196 

Chlorides,     deposited     from     de- 
scending waters 285 

in  oxidized  zone 286 

Chlorine,  as  solvent 240 

in  dry  climates 284 

in  minerals 109 

in  vapors  from  granite 17 

Chloritic  schist 3 


INDEX. 


315 


Page 

Chrome  iron.     See  chromite. 
Chromite,  magmatic  segregation.    106 

relation  to  basic  rocks. 113 

Chromium  deposits '. 113 

Chromium   oxide,    magmatic  dif- 
ferentiation      245 

Chromium  stains  on  outcrops  .  .  .    302 

Church,  John  A.,  cited 168 

Cinnabar,   deposited  from  fuma- 

roles 17,64,110 

Monte  Amiata,  Italy 63 

Clarke,  F.  W.,  cited 12 

Clays,  composition  of 8 

uses 8 

Clements,  J.  M.,  cited 95 

Cleavage,  distinction  from  strati- 
fication   32,  33 

Cleavage  planes,  denned 32 

Climate,    relation   to   ore-deposi- 
tion   273,  274,  275,  276 

Clinometer 135,  136 

Clough,  C.  T.,  cited 3 

Coal,  age  of 36,  66 

association  with  certain  stra- 
ta        69 

formation 66 

shales 73 

Cobalt,  Mansfeld,  Germany 70 

Colorado,  Aspen 

169,  170,  202,  203,  248,  308 

Bassick  mine 110 

Cripple  Creek 89,  196,  285 

Devonian 25 


Gunnison  region 

L'eadville 

Leadville  and  Aspen  .... 
Red  Cliff  district 


.    191 

.    308 

.      63 

268 


Rico 54,  171,  172,251 

Silver  Cliff 242 

Ten  Mile  district 173 

uranium  and  vanadium  .       .261 

Colors  of  gold 218 

Columbia  River  basalt 98 

Columnar  structure 175 

Concentration,  by  specific  gravity     21 
by  waters,  subsequent  to 
magmatic  segregation.  ...      12 

from  solution 267 

of  ores 20,  21 

of  valuable  elements 9 

Conglomerate,  defined 28 

gold-bearing 65,  226,  227 

oil-bearing  .  .  . 


passing  into  sandstone  . 


54 


suitability  for  ore-deposition.     28 

Conifers,  defined 50 

Conjugated  fractures 179 

Connecticut,  tungsten  mine  .    ...    108 
Contact-metamorphic     ore- de- 
posits  16,  107,  108,  245 

Contact-metamorphism 16 

Contraction  during  alteration  .  ..    256 

Contraction  in  rocks 175,  182 

Copper,  arsenic  compounds  with.    267 

association  with  iron 300 

Butte,  Mont 116 

in  igneous  rocks 100 

in  muds 58 


Page 
Copper,  in  Permian  strata. .  .70,  71,  72 

in  red  sandstones 70 

in  sea-water 71 

in  springs 242 

Lake  Superior 90 

precipitation  in  sediments  .  .  261 
preference  for  basic  rocks  ...  114 
replacing  plant  remains  ....  72 

secondary  sulphides 282 

stains,  on  outcrops 302 

veins,  Cornwall 10,  185,  296 

zone  of 268 

gossan 258 

Traversella,  Italy. 59 

Copper  carbonate,  formation  of .  .    286 

Sierra  Oscura 72 

Copper  minerals,  superficial  alter- 
ation      282 

Copper  oxide,  formation 286 

Copper  sulphide,  oxidation 286 

Cornwall,  copper  and  tin 296 

veins 10 

vein  system 1 85 

Correlation  of  strata 57 

Corundum 8 

magmatic  segregation 106 

use  of 106 

Cretaceous,  coal  in 66 

fossils  of 40 

Cretaceous  period :      37 

Crinoids,  age  indicated  by 43 

defined 45 

Cripple  Creek,  Colorado 89,  196 

Cross-sections,  construction.  .143,  144 

Crust,  movements  of 1 

Crustaceans,  defined 47 

Gratification    190 

Cryolite 8 

Cryptogams,  defined 50 

Crystallization  during  metamor- 

phism 6 

in  granite 13 

in  igneous  rocks 100 

Crystalline      structure,      igneous 

rocks 80,81 

Crystals,  growth  of 80 

Cuba,  copper 258 

iron  ores  , 118,119 

serpentine 114 

Cuprite 286 

Cycads,  defined 50 

Cycles  in  rock-formation 7 


Debris,  surface - 130,  131 

Decomposition  of  rocks,  guide  to 

prospector 303 

Deformation,  effects 126 

De  La  Beche,  cited 76, 185 

Depth,  changes  in  richness.^ .  ^  ^ 

changes  in  value 248,  299 

of  ore-deposition 254 

Descending  waters,  deposition  by 
22,  175,  189,  270,  271,  273,  279,  289 
ore-deposits,  changes  in 

depth      293 

ore-deposits,  criteria.  289,290,  291 


316 


INDEX. 


Descending  waters,  sulphide  depo- 


Page 


sition  by  .  . 
Devonian,  fossils  of 

gold-bearing  conglomerates 

gold  in 

in  Colorado 

rocks,  how  distinguished .  .  . 

Devonian  period 

Diabase,  alteration 

Diabasic  porphyry,  denned  .... 

rocks,  transitions 


287 
38 
65 

62 
55 
43 
37 
91 
86 
94 


gold  and  silver  in  ......    101 ,  102 

metallic  minerals  in 1 00 

segregation  of 10 

transition  to  diorite 95 

Diabasic  rocks,  defined 85 

transitions 94 

Diamond  placers 232 

Differentiation  of  dike 95 

Diller,  J.  S.,  cited  on  nickel  ores.  .      12 

Dike,  defined 96 

of  ore 117 

veins  formed  along 203,  204 

Dinosaur,  age  of 39 

defined 49 

Diorites,  alteration 91 

connection  with  ores 59 

gneiss 33 

quartz-bearing 59 

transition  to  diabase 95 

Dioritic  P9rphyry,  defined 86 

transitions 94 

Dioritic  rocks,  defined 85 

Dip,  defined 121 

how  recorded 134,  138 

reading  of 136 

Disintegration,  surface 257 

Dislocation  of  fault,  defined.  ...      153 
Displacement  of  fault,  defined.  .      153 

Doelter,  cited 240 

Dolcoath  mine,  Montana 16 

Dolerite,  defined 90 

Dolomite,        distinction        from 

marble 31,32 

origin  of 30,  31 

Dolomitic  marbles 31 

Dome 148 

Douglas,  James,  cited ,  .    309 

Drift 131 

Dynamic  geology,  definition  ....    120 

E 
Earthquakes,  fractures  produced 

by 178 

Economic  geology,  scope 8 

Egleston,  T.,  cited 240 

Elements,  in  the  earth 8 

segregation  in  molten  masses       9 

Elkhorn  district,  Montana 16 

Elongation  of  pebbles  in  conglom- 
erate          3 

Emery 8 

Emmons,  S.  F.,  cited 173,  268,  285 

Enargite 288 

England,  Derbyshire 76 

vein  systems 185 

Enrichment  in  oxidized  zone  ....    267 
Enrichment,  sulphide 269 


Page 
Eocene,  oil 69 

placers. 212,  213,  226 

Epidote  as  alteration  product  .  .  .   304 

in  contact  metamorphic  de- 
posits      108 

Equiseta,  defined 51 

Erosion,  defined 126 

gap  in  sediments 52,  53 

work  of 98 

Eruptions,  producing  fractures  ..  178 
Eureka,  Nevada,  age  of  ore  ....  63 

cave-deposits 75 

Evaporation,  ore-deposition  by.  .  261 
Expansion  during  alteration  ....  256 
Extrusive  rocks 95 

advantages      for     ore-depo- 
sition     Ill 

connection  with  hot  springs .    Ill 

defined  .  .  ,98 


False  bottom,  tin  placers 232 

False  saddle 290 

Farish,  J."  B.,  cited 251 

Faults,  accurate  measuring 1 50 

as  shown  in  vertical  cross- 
sections 149 

bedding.  .  . 158,  159 

compensating 124,  125 

definition 120 

dying  out  of 181 

estimation  of :  .  .  .    148 

existence  shown 150 

how  detected 139 

in  heterogeneous  rocks  . .    .• .    151 

in  homogeneous  rocks 1 50 

means  of  measuring 151 

measured  by  displacement  of 

strata 149 

normal 12,  13,  24 

ore-deposition  on 169,  172 

post-mineral 203 

pre-mineral 203 

relation  to  topography 

128,  307,  308 

reversed 123,  124 

Fault-breccia,    indicating    move- 
ment      152 

Faulted    district,    suitability    for 

ore-deposition 308 

Faulted  faults 200,  201,  202 

Faulting,  different  periods  of .  200,  202 

effect  upon  values 197 

reversal  of 200 

Fault-movement,  computation  of  157 
fractures  of 160,  161,  162 

Fault-scarp 128,  129, 130 

reversed 1 29 

simple 129 

Fault-zone,  nature  9f 170,  171 

ore-deposition  in  .  .  .170,  171,  173 

Feldspar,  altered  to  sericite 203 

in  gneiss 33 

in  igneous  rocks 

85,86,87,88,89,90 

in  veins 14,  15,  204 

Felsite,  defined 90 

in  Wales  . .  . .     91 


INDEX. 


317 


Page 

Ferric  sulphate  as  solvent ....  240,  278 
Ferro-manganese  minerals,  metals 

in 113 

Filled  deposits 190 

Fishes,  defined 48 

Fissility  in  shales 30 

Fissures,  cause  of 182,  183 

extent 298 

near  surface 189 

open 182 

ore-deposition  in 184,  185 

origin 184 

Fissure-eruptions,  lavas 98 

Fissure-filling  by  ores 23 

Fissure  vein 172,  191 

Flow-banding,  igneous  rocks  ....      80 
Flow-structure,  igneous  rocks  ...      82 

Fluorine,  in  minerals 109 

in  vapors  from  granite 17 

Fluorite,  association  with  tin.  .  .  .   300 
in  contact  metamorphic  de- 
posits.      107 

in  tin  veins 113 

Folding,  effect  upon  values 197 

Folds,  close 121 

connection  with  ores  .    .....    194 

how  joined 120 

kinds  of 121,122 

open 122 

ore-deposition  in 164,  165 

overthrown 122,  123 

relation  to  topography 128 

three  dimensions  of 148 

Foraminifers,  age  of 40 

defined 44 

Formation,  meaning  of  term  ....      27 

Forty  Mile  creek,  Alaska 218 

Fossil  placers 226,  227 

Fossils  as  evidence  of  geologic  age     35 

as  guide  to  age 42 

changed  tc  ore 248 

in  shales 73 

significance  of 27 

stretching  of 33 

use  in  identifying  ore-beds  .  .      68 

Foster,  C.  Le  Neve,  cited 68 

Fractures,  compound 180 

conjugated 179 

course  of 179 

definition 177 

dying  out  of 181 

imbricating 182 

in  different  strata 181 

in  hard  strata 75 

ore-deposition  along  185,  186,  243 

origin 177,  1 78 

reversal  of 199 

subsequent  to  veins 198 

Fracture-zones,  influence  on  ore- 
deposition 193 

Fragmental  rocks 26 

Friction  breccia 92 

Free-gold  in  outcrops 278,  302 

Fumaroles,  change  to  hot  springs.     18 

Monte  Amianta,  Italy 64 

ore-deposition  by.  .  .  .  17,  109,  110 
Fumarolic    action,     confined    to 

extensive  rocks  .  .  .111 


Fundamental  igneous  rocks 95 

connection  with  ore-deposi- 
tion    Ill 

exposure  of 98 

G 
Gabbro,  altered  to  greenstone  ...      10 

containing  ore 11,  105 

denned 90 

Galena,  deposited  in  mine  work- 
ings        64 

in  coal-shales 73 

zone  of 268,  280,  281 

Galicia,  oil 69 

Gangue  minerals 186 

Gap  mine,  Pennsylvania 11 

Garnet  in   contact  metamorphic 

deposits 108 

in  gabbro 117 

in  metamorphic  rocks 17,  59 

in  placer  gravels 210,211 

Garnet-schists 34 

Gas,    natural,    association    with 

certain  strata 69 

in  anticlinal  folds 166 

ore-deposition  by 

39,  106,  107,  108,  109 

origin 301 

Gasteropods,  defined 46 

Geologic  age,  length  of 35 

Geologic  periods 37 

association  with  certain  ores.     62 

how  named 36 

Georgia,  saltpeter 262 

Germany,  Kupferschiefer 70 

oil 69 

Pyippoldsau  and  Kissingen  .  .   242 

Stassfuth 310 

Gilbert,  G.  K.,  cited 2 

Glaciers,  connection  with  placers 

211,212,213 

effect  of 211,212 

erosive  power 131 

Glacial  period 41 

Glass,  volcanic 80 

Glassy  igneous  rock,  defined 85 

Glauconite 264 

Gneiss 3,  33,  108 

derivation    from    igneous 

rocks 34 

kinds  of 34 

Gneissic  structure 6 

defined 34 

Gold,  concentration  at  surface.  .  .  206 
concentration  in  gravels  ....  208 
concentration  in  surface 

water 21 

enrichment  by  oxidation  .  .  .    266 

formation  of 206 

geologic  pei  iods  found  in  ...     62 

in  arsenopyrite 280 

in  bay  mud 72 

in  contact  metamorphic  de- 
posits       17 

increase  by  oxidation 279 

in  granitic  quartz  veins 15 

in  igneous  rocks 1 00 

in  muds 58 


318 


INDEX. 


Page 

Gold,  in  pyrite 300  301 

in  quartz  veins 301 

in  sea-water 71,  102,  219 

in  serpentine 114 

in  springs 242 

in  valley  placers 216 

precipitated  by  pyrite.  .  .  .  77,  251 
precipitation   by   organic 

matter 264 

precipitation  in  placers 207 

precipitation  in  sediments  .  .    260 

residual  deposits 266 

rooted  deposits 233,  234 

solubility  9! 206,  240,  279 

Gold-quartz  veins,  Otago 194 

possible  origin 15,  107 

Goodrich,  H.  B.,  cited 218 

Gossan 258,  282 

Granite,  connection  with  tin  . .  17,  113 

giant 92 

gold  and  silver  in 101,  102 

in  Cornwall 10 

metamorphic  influence  of .  .  .      16 

replacement  by  ores 116 

replacement  of 248 

segregation  in 10 

transition  to  quartz  veins.  .  13,  15 

Granitic  porphyry,  denned 86 

Granitic  porphyry  rocks,  transi- 
tions       94 

Granitic  rocks,   connection  with 

tin- veins Ill 

defined 85 

replacement  of 250 

transitions 94 

Granodiorite,  defined 168 

Grant,  U.  S.,  cited , .     91 

Granular  igneous  rocks,   connec- 
tion with  ore-deposition  . .   109 

denned 84,  85 

Graptolites,  age  indicated  by ....     43 

defined : 45 

Gravels  in  river  valleys.  „ 25 

Great  Lakes,  crustal  movement .  .       2 

Greenland,  native  iron 105 

Greenstones  in  Cornwall 10 

defined 91 

origin 10 

Greisen,  defined 304 

Grit,  defined 28 

Groundmass,  in  igneous  rocks  ...     84 

Ground-water,  depth  of 238 

level 237 

source  of 237 

Guiterman,  Franklin,  cited 269 

Gulch  placers 214,  215 

Gunn,  W.,  cited 3 

Gymnosperms,  defined 50 

Gypsum,  associated  minerals. ...   301 

beds 58 

deposition  at  surface 25 

deposition  in  lakes 260 

in  Permian  rocks 68 

in  red  sandstone  strata 70 

residue  from.  .  .     74 


H 

Hade,  use  of  word 121 

Hague,  Arnold,  cited 81 

Hayes,  C.  W.,  cited 114,  119 

Haworth,  E.,  cited 195 

Heave  of  fault 162 

Heavy  spar 300 

Hematite,  Cuba 119 

deposited  by  fumaroles 

17,  110,  245 

Hematite,  Piemonte,  Italy 59 

Hess,  W.  H.,  cited 263 

Hill,  J.  B.,  cited 10 

Hinxman,  L.,  cited 3 

Hobbs,  W.  H.,  cited 108 

Hopkins,  T.  C.,  cited 270.  271 

Horn  silver 283,  286 

Hornblende,  containing  metals  .  .    100 
in  contact  metamorphic  de- 
posits     108 

in  gneiss 33,  108 

in  igneous  rocks . 82,  86.  87,  88,  89 

in  rocks 7 

in  tin  veins 113 

Hornblende-schist 31,  34 

Home,  J.,  cited 3 

Hornstone,  definition 59 

Hot  Springs,  connection  with  ex- 
trusive rocks Ill 

connection       with      igneous 

rocks 103 

connection  with  ore-deposits 

103,  104 

cooling  of 18 

deposition  of  minerals 63 

mineralization  by 59 

origin 18,  20,  103 

substances  contained 242 

Hot  waters,  producing  ore-depo- 
sition       19 

Howitt,  A.  W.,  cited 14 

Hydatogenic  deposits 107 

Hydrogen  sulphide  in  waters  ....    240 
Hydrogen  sulphide.  See  sulphur- 
etted hydrogen. 


Ichthyosaurus,  age  of  . 40 

defined 49 

Idaho,  basalt 98 

De  Lamar  district 204 

Silver  City  and  De  Lamar  . .  .  303 

Igneous  rocks,  cause  of  fluidity  ..  13 

characters  of 4 

classification 82,  83 

connection  with  hot  springs  .  103 
connection  with  ore-deposits 

99,  103 

cooling 94 

crystalline  structure 80 

defined 79 

derived  from  metamorphic.  .  5 

distinguishing  characters  ...  81 

forms  of 95 

fractures  in 181 

glassy  form 80 

mapping  and  sectioning  ....  147 

metamorphism  of 5,  6 


INDEX:. 


319 


Page 

Igneous  rocks,  naming  of 83 

origin 4 

stimulating  ore-deposit  ion  .  .    112 

textures 94 

transformation  to  sediments  .       6 

transitions 93,  94 

water  in 13 

Ilmenite  in  igneous  rocks 86,  100 

Imbricating  fractures 182 

Impervious    clay,    forming    pay- 
streak  232 

Impervious  strata,  relation  to  ore- 
deposition.  .73,164,165,166,209,  253 
Impregnation  deposits,  carnotite.  262 
Impregnation  of  rocks  by  ore.  ...      '. 

Indicators,  Ballarat 77 

Intersections,  ore-deposition  at.  .    169 
Intersections,  principle  of.  .  .196,  251 

Interstitial  filling  by  ores 23 

Intrusions,  producing  fractures.  .    178 
Intrusive  igneous  rocks,  defined .  95,  96 

Intrusive  mass,  defined 96 

Intrusive  ore-bodies 117 

Intrusive  rock 108 

advantages    for    ore-deposi- 
tion      112 

ores  derived  from 59 

subsequent  to  ore-deposition    118 

Iodine  as  solvent 240 

Iron,  association  with  copper.  .  .  .    300 
connection  with  basic  rocks . 

113,114 

deposits    by    descending 

waters 270 

derived  from  glauconite  ....    264 

in  bays 58 

in  diabase 10 

in  igneous  rocks 99 

in  sinter 254 

in  waters 241 

Lake  Superior 291,  292 

magmatic  segregation 105 

native 105 

precipitation      by      organic 

matter 264 

residual  deposits 266 

relative  importance 8 

rooted  deposits 234 

segregation  of 10 

zone  due  to  surface  waters  .  .    269 

Iron  cap 258 

Iron  carbonate 266,  271 

Iron  ores,  Cuba 118,  119 

Lake  Superior 61 

Piemonte,  Italy 59 

Iron  oxide 110 

deposited  from  fumaroles.  .  .      17 
magmatic  differentiation  of.    285 

Iron  sulphate  as  solvent 268 

auriferous 268 

Italy,  Monte  Amianta,  ores     ....      63 
Piemonte,  iron    and  copper 

ores 59 

sulphur 110 


Jasperoid 304,  305 

Jenny,  W.  P.,  cited 64,  74 


Page 

Joints,  columnar 174,  175 

defined 173 

dying  out  of 181 

how  studied 176 

ore-deposition  on.  ...  18,  175,  176 

origin  of 174 

relation  to  veins 175 

Jurassic,  gold  in 62 

oil  in 69 

origin  of  term -. 36 

Jurassic  period 37 

fossils  of 40 

Juvenile  springs 18 

K 

Kansas,  lead  and  zinc 194 

Kaolin,    due    to    rock    decompo- 
sition   304 

Kemp,  J.  F.,  cited 

.  .  11,  70,  105,  114,  117,  238,  245,  280 

Kentucky,  saltpeter 262 

Klockmann,  F.,  cited 285 

Kuskokwim  river,  Alaska 54 


Labradorite 117 

Lakes,  ores  precipitated  in 260 

Lake-sediments 24 

Lake  Superior,  copper 90 

iron  ores 61,  291 

Lamellibranchs,  age  indicated  by.     43 

defined 46 

Lateral  separation  of  fault 

154,155,156,157,158 

Lavas,  defined 98 

flow-structure 79,  80 

nature 4 

Lead,  association  with  silver  .... 

association  with  zinc 299 

deposits,  Derbyshire 76 

in  igneous  rocks 100 

in  springs 242 

Mansfeld,  Germany 70 

veins,  Cornwall 185 

zone  near  surface 268,  281 

Lead  and  zinc  ores,  Missouri 195 

Lead  carbonate 267 

Lead  sulphate 267 

formation 286 

Lead  sulphide,  deposited  by  fu- 
maroles     110 

formation  in  mine  workings  .      64 
Leadville,  Colorado,  age  of  ore ...      63 

Le  Conte,  J.,  cited 41 

Leucite  in  igneous  rocks 89 

Lepidodendrids,  defined 51 

Level,  changes  of 2 

Life,  beginning  and  development 

of 35 

Lignites,  Alaska 66 

Limbs  of  fold 121 

Lime,  in  waters 25,  241 

replacement  by  silica ....  304.  305 
Lime  carbonate  (See  also  calcium 

carbonate) 30 

as  sinter 254 

change  to  lime  phosphate .  .  .    263 
deposition  at  surface 25 


320  INDEX. 

Page 
I.ime  carbonate,  in  dolomite  ....      30 

Lime  phosphate  .............  68,  109 

origin  ....................    263 

rooted  deposits  ...  .........    234 

Lime  sulphate  deposition  at  sur- 

face ...........  ........      25 

Limestones,  change  to  lime  phos- 

phate. .  .    ..............   263 

chemical  deposition  ........      25 

containing  cinnabar  ......  63,  64 

containing  ores.  ....  .......      59 

destruction  from  dolomite  .  31,  32 
fetid  .....  ...............    246 

69 
31 
16 

30,  263 
59 

selected  by  ore-deposition  .  . 
...................  75,76,78 

Limonite,  Monte  Amianta,  Italy  .      64 
Lindgren,  W.,  cited  ............ 

.  .  .  .99,  108,  125,  201,  204,  221,  222, 

224,  225,  266,  278,  295,  298,  300,  304 
Linked  veins  ..................    192 

Lithia  mica,  association  with  tin  .    300 
Lithium  in  springs  .............    242 

Longitudinal  sections  ..........    145 

Lotti,  B.,  cited  ................      64 

Lycopods,  defined  .............     51 


in  oil-bearing  strata 
magnesian 
metamorphism  of 
origin 
replaced  by  ore 


M 
Macedonia,  gold  ........  228,  229,  265 

Magmatic  differentiation  in  dike  .      95 
Magmatic  segregation  .......... 

......  10,  13,  104,  105,  106,  107 

Magnesia  in  diabase  ............      10 

Magnetic  iron  pyrite  (See  alse  pyr- 

rhotite).  .  ..............      11 

Magnetic  variation  ...........    135 

..  119 
..  117 
86,  100 
..  210 
.  .  113 
..  105 
..  31 
30 


Magnetite,  Cuba 

dikes  of 

in  igneous  rocks 

in  placer  gravel 

in  tin  veins 

magmatic  segregation. 

Magnesian  marbles 

Magnesium  in  waters ..... 
Magnesium  carbonate  in  dolomite     30 
Magnesium    salts,    deposition    in 

lakes 260 

Malachite 286 

Malay  Peninsula,  tin 113 

Malvern  Hills,  metamorphism.  .  .        6 

Mammals,  denned 48 

Mammoth,  period  of 42 

Man,  period  of  existence 42 

Manganese,  deposits  of 260 

in  igneous  rocks 99 

in  oceans 58,  260 

Manganese  oxide,  deep  formation  245 

Manner  of  ore-deposition 23 

Mansfeld,  Germany,  ores 70 

Mapping,  economic  results 145 

geological,  how  done 138 

of  igneous  rocks 147 

Maps,  how  made 137 

use  of... 136,137 


Page 

Marble,  dolomitic 31 

gold  and  silver  in 102 

origin 16,30 

Massachusetts,  Cape  Ann.  .  .  .  182,  183 
Mechanical     action     of     surface 

waters 21 

Mechanical  agencies,  ore-conceu- 

tration  by 236 

Mercur,  Utah 179 

Mercury,  Mansfeld,  Germany.  ...     70 

Mercury  sulphide 110 

deposited  from  fumaroles.  .  .      17 

Italy 63 

Mesabi  range,  Minnesota  .  91,  291,  292 

Mesozoic  era 37 

fossils  of 39 

vein  formation 63 

Metallic  minerals  deposited  from 

fumaroles 17 

Metalliferous     veins,     period     of 

f9rmation 63 

Metals   in   ferro-magnesian   min- 
erals     113 

in  rocks 7,  10,  99,  100 

Metals,  rarer,  occurrence 9 

Metamorphic  processes  connected 

with  ores 7 

Metamorphic  rocks,  banding  of  .  .        4 

denned 3 

derived  from  igneous 5 

origin  of  characteristics 4 

transformation  to  igneous  .  .        5 
transformation  to  sediments        6 

Metamorphism,  contact 16 

of  conglomerates 3 

of  sediments 2,  16 

Meteoric  waters,  heating  of 20 

producing  ore-deposition  ...      19 

Mexico,  Pachuca 192,  193 

Mica,  containing  metals 100 

in  contact  metamorphic  de- 
posits     108 

in  quartz  veins 14 

in  rocks  .  .  .7,  33,  85,  86,  87,  88,  89 
Mica  schist,  associated  with  ores.      59 

origin 3 

Michigan.  Crystal  Falls 95 

Microscope,  petrographic 82,  83 

Migration  of  outcrops 139,  140 

estimation  of 141,  142 

Mine-workings,  ore-deposition  in 

20,  64,  288 

Mineral  wax 301 

Mineral  zones  near  surface 268 

Mineralization  by  vapors 16 

Mineralizing  solutions,  chemicals 

175,  176 

Minerals,  associations  of.  .58,  299,  301 
Mingling  of  ore-bearing  solutions 

-. 251,252 

Minnesota,  Mesabi  range  .91,  291,  292 

Miocene  placers 226 

Mispickel,  gold  in 206 

Missouri,  Belleville 73 

lead  and  zinc 194 

Joplin  district 64 

origin  of  dolomite 31 

Mollusks,  defined 45 


INDEX. 


321 


Page 

Molybdenum,     connection    with  Olivine,  alteration  of " 

silicious  rocks 113  containing  metals 100 

Monocline,  defined 126  in  igneous  rocks 12,  86,  88 

Monazite  in  placers 232  nickel-bearing 12 

use  of 232       Omeo,  Australia,  veins 14 

Montana,  Butte  district 116,  288       Ordonez,  E.,  cited  . 192 

Dolcoath  mine 16  Ore-bearing  strata,  dimensions  of.     61 

Elkhorn  mine 166       Ore-bodies,  defined 235 

fossil  placers. 227  shrinkage  of 76 

Monte  Amianta,  Italy,  ores 63  Ore-concentration,  conditions  . . .   243 

Monte  Cristo .  .  63,176,250,279,280,  281  Ore-dep9sition  by  hot  springs  ...     63 

Monte  Cristo,  Washington,  age  of  by  juvenile  waters 19 

ore 63  by  release  of  pressure  . .  .  253,  254 

Mountains,  fayorab'eness  for  ore-  chemical  agencies 236,  237 

deposition 306,  307  favorable  conditions  for  ....    112 

origin 127,  128  in  mine-workings 20 

Movements    subsequent    to    ore-  manner  of  . 22,23 

deposition 197  mechanical  agencies 236 

Murchison,    Sir    Roderick,    men-  on  lowered  temperature  .  253,  254 

tioned 62  recent 288 

Muscovite  in  contact   metamor-  Ore-deposits,  connection  with  hot 

phic  deposits 108  springs 103, 104 

in  greisen 304  connection    with    igneous 

in  quartz  veins 14,  15  rocks 99 

in  tin  veins 113  connection    with    rock    dis- 
turbances      199 

N  contact  metamorphic 16 

Native  metals 244  date  of  formation 65 

Native  silver 267  depth  of  formation 254 

Aspen 248       Oregon,  basalt 98 

Neocene  placers 224  Blue  Mountains 221,  266,  278 

Nepheline  in  igneous  rocks 89  nickel  ores 12 

Nevada,  De  Lamar  mine 285  Ores,  association  with  certain  geo- 

Eureka  district 63,  75,  308  logic  periods 62 

Pioche 308  changes  in  depth 12 

silver  chloride 287  contemporaneous  with  strata     69 

Silver  Peak 15  deposited      after      volcanic 

Steamboat  Springs 63  eruptions 63 

Tertiary  fossils 41  deposited  by  surface  evapo- 

trachyte  in 89  ration  .  . 261 

New  Jersey,  Franklin  Furnace. 244, 245  derived  from  intrusive  rock.     59 

New  Mexico,  copper 72  re-deposition  of 189 

silver  chloride 287  selective  precipitation 115 

New  York,  Adirondack*, 105       Ore-shoots  .' 195, 196 

New  Zealand,  Otago Organic  acid  in  rock-weathering.    257 

....  194,  212,  213,  224,  226,  249  Organic  matter,  precipitation  by 

Nickel,  Gap  mine 11  ......  .71,  72,  73,. 245,  246,  264,  288 

in  igneous  rocks 12,  100,  106       Organic  minerals,  origin 301 

Mansfeld,  Germany 70       Organic  sediments 25,  26 

Oregon 12       Otago,  New  Zealand 194,  212,  213 

segregation  of 10       Outcrop,  ore-bodies 20,  302 

stains  on  outcrops 302  migration  of  ...  139,  140,  141,  142 

Nitrates,  cavern  formation 262  of  rocks 130, 131 

Nitric  acid,  in  rock-weathering  .  .    257  selection  of 138 

organic  origin 263       Orthoclase  as  gangue  mineral 204 

Novarese,  V.,  cited 59       Osseous  fishes 49 

Oxidation,  depth  of 279,  280 

O  guide  to  prospector 301 

Obsidian 80  of  ores 257,  258 

Oceans,  ores  precipitated  in 260  relation  to  ore-concentration 

Offset  of  fault 159, 162, 163  .272,  273 

Oil,     association     with     certain  zone  of 257 

strata 69  Oxides,     association     with     sul- 

in  anticlinal  folds 166  phides 245 

Oil  deposits,  preference  for  cer-  deposited  by  descending  wa- 

tain  geologic  periods 67  ters 285 

Old  placers 222,  223,  224,  225,  226  formation  at  surface 256 

Oligocene,  oil 69  formation  at  depth 244 


322 


INDEX. 


Page 
Oxidized  zone,  concentration  of 

metals  from 272 

depth  of 259 

enrichment  in 266,  267 

erosion  of 272 

minerals  in 286 

Ozark   uplift,    influence    on   ore- 
deposition 194 

Ozocerite,  origin 301 


Pachuca,  Mexico 192 

Paleozoic,  oil  in 69 

veins  in 63 

Paleozoic  era 37 

Pay-streak  in  placers ....  208,  209,  232 

Peach,  B.  N.,  cited 3 

Peat-swamps,  Alaska 66 

Pebbles,  stretching  of 33 

Pegmatite,  denned 92 

formation 106,  109 

Pennsylvania,  Gap  mine 11 

iron  ores 270 

Penrose,  R.  A.  F.,  cited 

113,  196,264,288 

Peridotite,  alteration 12,  114 

composition 12 

containing  ore 11 

Peridotitic  rocks,  alteration 92 

containing  chromium 113 

denned 86 

Permian,  copper  in 70,  71 

minerals  in 67 

Perpendicular  separation  of  fault 

155,  156,  157,  158 

Petroleum,  origin 301 

Petroleum-bearing  strata 69 

Phenocrysts,  defined 84 

Phenograms,  defined 50 

Phillips,  J.  A.,  cited 71 

Phonolite,  defined 89 

Phosphate  of  lime,  England  and 

France 68 

in  veins 109 

origin 263,266 

rooted  deposits 234 

Phosphoric  acid,  organic  origin  .  .    263 

Physiography,  defined 306 

relation  to  mining 306 

Piemonte,  Italy,  iron  and  copper 

ores 59 

Placers,  age  of 65 

beach 218,219 

bench 221 

black  &and 210 

broad  valley 214 

defined 205 

diamond 232 

Eocene 212,213 

false  bottom 208,  209 

fossil 226,  227 

gold 207,214,215 

mechanical  origin 208 

new  generations  of 227 

old 222,  223,  224,  225,  226 

origin 205,  207 

pay-streak 208,  209 


Placers,  platinum 229 

precipitation  of  gold  in  .  .  206,  207 

reconcentrated 227,  228,  229 

ruby  sand 210 

second  paynetreak  in 209 

solution  of  gold  in 263 

Tertiary 226,  228,  229 

tin  ..  . 230,231,232 

volcanic  capping  of 223,  224 

Plains,  ore-deposits  in 306,  307 

Plant  remains,  age  indicated  by .  .      43 
Platinum,   concentration  in  sur- 
face water 21 

in  rocks 100,  114 

placers 229 

Pleistocene,  veins  in 63 

Porous  strata,  effect  on  ore-depo- 
sition.  74,253 

ore-deposition  in 243 

Porphyritic  crystals 84,  91 

Porphyritic  rocks,  defined  .  .  84,  86,  87 

transitions 94 

Porphyritic  structure 84 

Porphyry 173 

incorrect  use  of  term 303 

quartz 88 

Portugal,  pyrite  deposits 285 

Posepny,  F.,  cited 242 

Post-mineral  faults 202,  203 

Potassic  chlorate  as  solvent 240 

Potassium  nitrate 262 

Potassium    salts,    deposition    in 

lakes  :.., 260 

Potassium  sulphate 110 

Pottery,  materials  for 8 

Pre-Cambrian      auriferous     con- 
glomerate      227 

Pressure,  decrease  of 253,  254 

Primary  association  of  strata  and 

minerals 58 

Primary  ores 269 

Primary  ore-beds,  secondary  con- 
centration in 60 

Principle  of  intersection 196,  251 

Protozoans,  defined 44 

Pseudomorphs 248 

Pterodactyls,  age  of ,      40 

defined 49 

Pumice,  defined 93 

Pyrite,  alteration  to  chalcopyrite.  250 

as  precipitant  of  gold 77 

auriferous 300,  301,  302 

deposited  by  fumaroles 110 

formation  in  shale  . 77 

gold  in 206 

in  altered  rock,  significance.    303 

in  coal-shales 73 

in  contact  metamorphic  de- 
posits     108 

in  igneous  rocks 100 

Monte  Amianta,  Italy 64 

Piemonte,  Italy 59 

precipitant  of  gold 251 

Pyroxene,  alteration  of 92 

containing  metals 100 

in  igneous  rocks  .  86,  87,  88,  89,  90 

in  metamorphic  rocks 17 

in  periodotite 12 


INDEX. 


323 


Page 

Pyrrhotite,  copper-bearing 285 

Gap  mine 11 

in  igneous  rocks 86,  100 

nickeliferous 106 

Q 

Quartz  in  rocks 7,  33,  85,  86,  87 

Quartz-feldspar  rocks,  origin  ....      13 

Quartz  porphyry,  defined 88 

Quartz  vein,  auriferous 77 

distinction  from  quartzite  .  . 

magmatic  origin 106 

outcrop. 302 

possible  origin 14 

transitions  into  granites  ....      13 

Quartzite.  defined 28,  29 

fractures  in 181 

origin  of 16 

replacement  of 248 

suitability  for  ore-deposition.     78 

Quaternary,  fossils  of 42 

origin  of  term 36 

veins  in 63 

Quaternary  period   37 

R 

Radiates,  defined 45 

Rainfall,  relation  to  ore-concen- 
tration  273 

Ransorne,  F.  L.,  cited .  171,251,252,262 

Rarer  elements  in  rocks 7 ' 

Rarer  metals,  occurrence 9 

Realgar,  deposited  by  fumaroles  .    1 10 

Monte  Amianta,  Italy 64 

Recent  ore-deposition 288 

Reconcentrated  placers  .  227,  228,  229 
Red  sandstones,  connection  with 

minerals 70 

copper  in 72 

Replacement  of  andesite 250 

of  granite 116,  250 

of  hornblende 116 

of  lime 304,  305 

of  limestone 39,  76 

process  of 247 

of  rock 23,  248 

of  schist 194 

Replacement  deposit,  marks  of.  . 

247,248 

Reptiles,  classified 49 

defined 48 

Residual  deposits 233,  234,  278 

manganese 260 

origin 266 

Rhizopods,  defined 44 

Rhode  Island,  magnetite 105 

Rhyolite,  alteration 91 

defined 87 

glassy 81 

transitions 94 

Ribbon  structure.  .  .  191,  199,  200,  26 1 

Rickard,  T.  A.,  cited 77,  164, 

165,   167,   171,   172,   188,   194,  212, 
213,  224,  226,  249,  251,  290,  291,  309 

Rico,  Colorado 74,  171,  172 

Riddles,  Oregon,  nickel  ores 12 

Rigid  stratum,  selected  for  ore- 
deposition 75 

Rim-rock  (in  placers) , ,  .  .    223 


Page 

Rivers,  change  of  bed 24,  222,  223 

River  sediments 24 

Rock,  defined  by  .  . 79 

Rock-forming  minerals 7 

Rohn,  Oscar,  cited 61 

Rolker,  C.  M.,  cited 231,  232 

Rooted  deposits 233,  234,  279 

Ruby 106 

sand  in  gravels 210 

silver 283,284 

Russia,  platinum 114 

Ural  Mountains  .  . .  .229,  230,  233 
Rutley,  F.,  cited 91 

S 

Saddle,  false.  .  .  . 290 

Saddle-veins 164,  165 

Salt,  associated  minerals 301 

deposition  of 260 

in  Permian 68 

in  red  sandstone  strata 70 

Salt-deposits     indicated    by    to- 
pography     309 

Salt  flats,  origin 261 

Saltpeter,  formation 262 

Sand  as  an  ore 8 

Sandstone,  connection  with  min- 
erals       70 

copper  in 72 

defined 28 

fractures  in 181 

gold  and  silver  in 101 

impregnation  deposits 262 

in  Triassic 55,  56 

metamorphism  into  quartz- 
ite       16 

metamorphism  into  schists .  .        3 

*     oil-bearing     69 

passing  into  conglomerate ...      54 

passing  into  shale 54 

selected  for  ore-deposition  .  .      73 

Sapphire 106 

in  tin  veins 113 

Saurians 49 

Scapolite  in  contact  metamorphic 

deposits 108 

in  veins 109 

Scheelite 108 

Schist 3 

defined 33 

derivation    from    igneous 

rocks 34 

derivation  from  sediments  .  .  3,  34 

kinds  of 34 

replacement  of 194,  248,  249 

selected  for  ore-deposition  .  .     73 

silicification  of 249 

Schistosity,  defined 34 

Schrader,  F.  C.,  cited. 215,  220 

Sea-shore,  concentration  of  ores 

on 21 

Sea-water,  gold  in 102,  219 

metals  in 71 

Secondary    association  of  strata 

and  minerals 58 

Secondary  concentration,   condi- 
tion dependent  on  ....  276,  277 
in  primary  ore-beds 60 


324  INDEX. 

Page 
Secondary  sulphide  enrichment .  .    269 

Secondary  sulphides 269,  282 

Sectioning  of  igneous  rocks 147 

Sections,  vertical,  construction  of 

143,144 

Sedimentary  ores 260 

Sedimentary    rocks,    advantages 

for  ore-deppsition 112 

association  with  certain  min- 
erals       68 

chosen  for  ore-deposition  ...     77 
derived    from    igneous    and 

metamorphic  rocks 6 

gold  and  silver  in 101,  102 

kinds 28 

physical  characters 26 

succession. 52 

Sediments,  chemical 25 

elevation  of 26 

formation  of .  . 24 

lateral  transitions 53 

organic 256 

transformation  to  hard  rocks     26 

Segregation  in  granite 10 

in  molten  masses 

magmatic 104,  105,  106,  107 

of  nickel. 11 

Sericite,  alteration  from  feldspar .   303 

Serpentine 92 

Cuba 114 

derived  from  peridotite 12 

Sgonnan  More,  metamorphism  . .        3 

Shales,  defined 29 

fractures  in 181 

iron  in 77 

metamqrphism  of 3,  16 

oil-bearing 69 

passing  into  sandstone 54 

selected  for  ore-deposition  .  .     78 

Shaler  N.  S.,  cited 182, 183 

Shallow  underground  waters  (See 

also  vadose  waters) 255 

Shearing  in  metamorphic  rocks  .  .   5,  6 

Shear  zones 193 

influence  on  ore-deposttion. 

193,194 

Sheet,  intrusive,  defined 96 

Sierra  Oscura,  New  Mexico 72 

Sigillarids,  defined 51 

Silica,  as  sinter 254 

deposited    from    surface 

waters 25 

in  granite 10 

Silicates,   decomposition  at   sur- 
face     256 

metallic 244 

Silicious  constituents,  concentra- 
tion of  13 

Silicious  dikes,  Cornwall 10 

Silicious  igneous  rocks,  ores  in  .  13, 102 
Silicious  rocks,   connection  with 

tungsten  and  molybdenum  113 

Silicon  as  an  ore 8 

Silification  near  veins 304 

Sill,  intrusive,  defined 96 

Silurian,  gold  in 62 

lack  of  coal  in 67 

fossils  of 38 


Page 

Silurian  period 37 

Silurian  rocks,  how  distinguished.     43 

Silver  and  gold  in  rocks 101,  102 

association  with  lead 299 

chloride 267 

decrease  by  oxidation 279 

deposited  by  fumaroles 110 

in  arsenopyrite 280 

in  igneous  rocks 100 

in  muds 58,  72 

in  sea-water 102 

Mansfeld,  Germany 70 

Peak,    Nevada,    gold-quartz 

veins 15 

precipitation  in  sediments  .  .    260 

solubility 279 

Silver-lead  deposits  in  limestone  .      76 

Sinter,  formation 254 

Slate,  defined 30 

Slickensides 150 

Slopes,  relation  to  ore-deposition . 

274,275,276 

Smith,  G.  O.,  cited 

186,  187,  255,  267,  305 

Soda,  formed  by  evaporation.  ...   261 

Sodic  carbonate  as  solvent 240 

Sodic  chloride  as  solvent 240 

Sodic  sulphide  as  solvent 240 

Sodic  sulphydrate  as  solvent  ....   240 

Solfataras,  Italy 64 

Solubilities,  concentration  accord- 
ing to 265 

Soluble    minerals,    indicated    by 

toppgraphy 309 

Spain,  pyrite  deposits 285 

Specular  iron,  deposited  by  fuma- 
roles     110 

Piemonte,  Italy 59 

Spencer,  A.  C.,  cited 119,  190,  191 

Springs,  hot.    See  Hot  Springs. 

Spurr,  J.  E.,  cited 

13,  54,  122,  129.  1 69,  170,  174, 

176,  180,  202,  217,  248,  250,  265,  281 

Stains,  mineral 302 

Stalactities  of  ore,  signification  .  .   292 
Stalagmites  of  ores,  signification  .    292 

Steam  from  lavas 15,  17 

Steamboat  Springs,  Nevada 63 

Stibnite,  Monte  Amianta,  Italy . .      63 

Stink-shales 246 

Strata,  association  with  valuable 

minerals 58 

contemporaneous  with  ores  .     69 

correlation  of 57 

identification     by     physical 

characters 55 

ore-bearing,  dimensions  of  . .     61 
persistence  of  characteristics.     55 
Stratification,  absence  in  igneous 

rocks 79 

distinction  from  cleavage  .  .  32,  33 

explained 27 

Stratified  rocks,  fractures  in 181 

veins  in 186 

Stratum,  defined 27 

selected  for  ore-deposition  . .     74 

Streams,  concentration  in 21 

Stream- works 230 


INDEX. 


325 


Page 

Stretching  of  pebbles  and  fossils  .      33 
Striae  on  fault-planes  .  .  .150.  151,  152 
Strike,  denned  ................    134 

how  recorded  ..........  134.  138 

reading  of  ................    135 

Structural  geology,  definition  .  ..    120 

Subsequent  fractures,  course  of  .  .    198 

in  veins  ..................    198 

substitution  of  ores  for  rock  .      23 
Suess,  Edouard,  cited  .........      18 

Sulpharsenides  ...............    267 

Sulphates,  deposited  by  descend- 

ing waters  .............    285 

derived  from  sulphides  .    ...    268 

reduced  to  sulphides  ......    270 

Sulphide  enrichment  ..........    269 

Sulphides,  association  with  oxides  245 
contemporaneous  with  oxides  284 
decomposition  at  surface  .  .  .    256 
deposition  of  .......  244,  245,  287 

secondary  ................    269 

Sulphur,    deposited    from    fuma- 

roles  .................  64,  110 

Monte  Amianta,  Italy  ......      64 

origin  .................  269,  270 

Sulphuretted  hydrogen  (See  also 

hydrogen  sulphide)  ......    246 

precipitation  by  ...........    251 

volcanic  .................      64 

Sumatra,  tin  ..................    231 

Superficial    alteration   of   copper 

ores  ...................    282 

Superficial  enrichment,  depth  of  . 

.............  .  ......  284,285 

Superposition  of  strata,  rule  of  .  .  .      56 
Surface  changes  ........  1,2,  222,  223 

fissures  near  ............  82,  183 

veins  formed  near  .........    189 

surface  slopes,  relation  to  ore- 
concentration  ...........    273 

Surface  waters,  effect  in  ore-depo- 

sition ............  20,  255,  256 

Swamps,  precipitation  of  ores  .  .      259 


Sweden,  magnetite 
Syenite,  containing  platinum.  .. 
defined 


gold  and  silver  in  ......  .10  ,102 


Syenite  gneiss 
Synclines 


105 

114 
89 


33 

121 
ore-deposition  in  ..........    164 


Talus 140 

Teliosts 49 

Telluride,  bismuth 17 

Tellurides 244 

deposited  by  fumaroles 110 

Temperature,  decrease  of  ....  253.  254 

Tennessee,  Ducktown 285,  288 

Tenorite 286 

Tertiary,  coal  in 66 

fossils  of 40 

oil  in 69 

origin  of  term 36 

placers  in 224,  226,  228,  229 

topography 224 

veins  in bo 

period 37 


Page 

Tertiary 224 

Texas,  copper 71 

limonite 264 

Thallogens,  defined 50 

Throw .  .  .159,  160,  161,  162 

of  fault 162 

Tin,  associated  metals 300 

concentration  in  surface  wa- 
ter        21 

connection  with  granite  ....    113 

Cornwall 296 

in  igneous  rocks 100 

in  sinter 254 

in  springs 242 

in  veins 109,  232 

Tin  oxide 109,  230 

Tin  placers 232 

Tin  veins,  alteration  of  rocks  near  304 
connection    with   granitic 

rocks HI 

Cornwall 10,  185 

formation 109 

origin 17 

Tintic,  Utah 186 

Titaniferous  iron 105 

Titanium  in  iron 113 

Tonalite,  Monte  Cristo 250 

Topaz,  association  with  tin .  .  .113,  300 
in  contact  metamorphic  de- 
posits     108 

in  veins 109 

Topography,  how  produced  . . 

relation  to  faults 128 

relation  to  folds 

relation  to  ore-deposition       .    273 
Total  displacement  of  fault  .  . 

153,154,157 

Tourmaline,  association  with  tin . 

231,300 

in  contact  metamorphic  de- 
posits      108 

in  veins 14,  109,  113 

Tower,  G.  W.,  Jr.,  cited 

186,  187.  255,  267,  305 

Trachyte,  defined 89 

ores  deposited  from 63 

Trap  rock,  defined 92 

Triassic,  coal  in 66 

conditions  favoring  ore-depo- 
sition       72 

copper  in 70 

fossils  of .'  .      39 

gold  in 62 

oil 69 

Triassic  period 37 

Triassic  rocks,  persistence  of  char- 
acteristics  55,  56 

Trilobites,  age  of 38,  42 

defined 47 

Troughs  of  synclines 164 

Tuff,  defined 91 

Tungsten,  connection    with    sili- 

cious  rocks 113 

in  tin  veins 113 

Turner,  H.  W.,  cited 15,  72 

U 
Uinta  range,  Utah 127 


326  INDEX. 

Page 

Unconformity  in  sediments  .....      53 
Underground     waters,     chemical 

work 22 

mechanical  action 21 

Uralite 101 

Ural  Mountains,  platinum  . .  .  229,  230 

rooted  gold  deposits 233 

Uranium,  formed  by  evaporation 

261,262 

Utah,  copper  in 70 

Horn  Silver  mine 283,  285 

Mercur 179 

Tintic 186,  255,  267,  305 

Uinta  range 127 


Vadose  waters,  concentration  by .      12 

Valley  gravels 25 

Valley  placers 214,  216 

Valleys,  origin 127 

Value,  changes  in  depth 298,  299 

Vanadium.formed  by  evaporation  262 

Van  Hise,  C.  R.,  cited 310 

Vapors,  deep-seated,  ore-deposi- 
tion by  108,  109 

forming  ore  deposits 17 

aqueous 16 

Vaughan,  T.  W.,  cited 114,  119 

Vegetable  kingdom,  divisions  ...      50 

Veins,  age  of 63 

banding 190,  191 

changes  in  depth 298,  299 

deflection  of 186,  188 

downward  extent 

294,  295,  296,  297 

dying  out  of 186 

enrichment  by  oxid&tion ....    297 
influence  on  topography.  .  .  .    308 

linked 192 

relation   between  horizontal 

and  vertical 297,  298 

relation  to  joints 175 

superficial 189 

transitions  into  granites  ....      13 

Vein-systems 185 

Vertebrates,  defined 48 

Vertical  separation 

157,  158,  159,  160,  161,  162 

Vesuvius,    fumarolic    ore-deposi- 
tion        17 

Virginia,  coal 66 

Vogt,  J.  H.  L.,  cited , 239 

Volcanic  breccia 93 

Volcanic  eruptions,  ores  deposited 

after .63 


Volcanic  glass 80^" 

Volcanic  pipes ll( 

Volcanoes,  eruptions 94 

W 
Wagoner,  Luther,  cited.  .  .72,  101,  21! 

Wales,  Caradoc \" 

Walls  of  vein,  multiplicity  of  ....    1' 

Washington 131 

Washington,  basalt ...      98 

Waters,  chemical  work  .  .  22 

effect  of 238;  -24* 

expelled  from  cooling  rocks  .      15 

in  rocks 13,  239 

mechanical  action 21 

ore-deposition  by 19/,  21,  22 

precipitation  from T.  .  .    243 

jlvent  power 241 


Watsfen,  T.  L.,  cited  . 


261 


Wax,  mineral  origin \    301 

Weathering,  zone  of 256 

Weed,  W.  H.,  cited 116,  166,  280 

White,  A.  A.,  cited *- r     127 

Winqfeell,  A.  N.,  cited 227 

Winchell,  H.  V.,  cited 288,  292 

Winslow,  Arthur,  cited 31 

Wit  water  srand  and  gold-bearing 

conglomerates 65 

Wplfram,  association  with  tin  . . .   300 

Wolframite 108 

Wolfram  minerals 108 

Wyssotzky,  N.,  cited 234 


Yellowstone  Park,  obsidian 80  . 


Zaitseff,  A.,  cited 230 

Zinc,  associated  with  lead 299 

in  igneous  rocks 100 

in  springs 242 

Mansfeld,  Germany 70 

zone  near  surface 268 

Zinc-oxide,  deep  formation 245 

Zinc  sulphide,  deposited  by  fuma- 

roles 110 

Zoisite  in   contact   metamorphic 

deposits 108 

Zone  of  crushing 249 

of  mineral  depositipn .  269,280,284 

of  secondary  sulphide 269 

of  weathering 256 

Zuber,  Rudolf,  cited 69 


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